One of the things a good chief financial officer does is keep the CEO from saying anything stupid.

That might mean shushing a boss who is about to give away too much insight on an upcoming quarter or, as happened with Cigna, coaching the CEO to please stop using entirely reasonable terms to discuss why it should merge, words like “dominant” and “market share.”

These are “sensitive” words to say out loud once federal regulators start looking at any deal, as they have with Cigna’s agreement to be acquired by Anthem Inc.

Sure enough, that conversation came up in a lawsuit the Department of Justice brought in late July to block the pending deal. It was one of two suits filed the same day to prevent the “big five” health insurers from completing deals that would shrink the group to the big three.

Aetna’s proposed purchase of Humana seems to have some life, but neither deal can be certain to close. That means the company with the dominant market share, getting all the benefits of being a lot bigger than rivals that the Cigna boss so clearly understood, is going to remain Minnetonka-based UnitedHealth Group.

The federal lawsuits described just how heated up the deal-making got that led to these mergers. The leaders of the companies suspected in early 2014 that there might be only one more big deal left that they could slide past the Justice Department. But what kicked off what the DOJ called “a bidding frenzy” was news last year that Kentucky-based Humana was exploring a sale.

In a two-month period, Anthem made several bids for Cigna, as described in the suit, while Cigna made two bids for Humana, UnitedHealth made bids for both Aetna and Cigna and, finally, Aetna made a bid for Humana. That last one is the first deal that actually came to fruition, a $37 billion merger agreement that got announced in July of last year. A few weeks later, Cigna agreed to a deal with Anthem valued at $54 billion.

While Aetna and Humana reached an agreement quickly, one of the things that stands out in the securities filings on the deal is the back-and-forth in negotiations over what got described as a “regulatory termination fee.”

Everyone seemed to know the Feds might not allow the merger. If Aetna really wanted this deal, it would have to take that risk and pay Humana a lot if the transaction didn’t close. Negotiators finally landed at $1 billion.

Anthem said it’s planning to fight back, and Aetna has said it likes its chances to persuade a court to let its deal with Humana close.

Aetna’s position seems to hinge on the notion that the market for private plans that administer Medicare benefits should properly be all the eligible beneficiaries, more than 50 million Americans. It can’t really matter, then, that its proposed merger partner, Humana, seems to have an awful lot of the 17.6 million Americans who now choose to get their Medicare benefits through a private plan.

This doesn’t seem like a winning argument. It’s a little like saying that there are a lot of parts that go into a new car so it couldn’t possibly matter that two of the handful of remaining providers of axles have decided to merge.

The Medicare Advantage market is more than the focus of this suit; it’s also a market niche that seems to nicely illustrate just what an industry leader UnitedHealth is. What’s at stake here is the government’s contention that Humana is already big enough without being combined with Aetna’s far smaller business.

The competitive landscape in Medicare Advantage varies a lot by state, but in 10 states Humana has the most members and it’s in the top three in an additional 19 states, according to a May report by the Henry J. Kaiser Family Foundation. Altogether it has about 18 percent of the market.

As big as it is, Humana still looks up in the Medicare Advantage market to UnitedHealth, with 21 percent of the overall market and the leading share in 19 states. And now, thanks to the feds, UnitedHealth may not have to worry that the No. 2 player will be allowed to get any bigger.

UnitedHealth has already been doing well enough without any unintended help from regulators. It’s recently been growing its share of Medicare Advantage, based on the same Kaiser study from last year, as Humana’s share has since slipped.

Revenue in the second quarter for UnitedHealth’s Medicare and retirement businesses grew nearly 14 percent, to $14.3 billion. The company has added nearly 500,000 to the total member count in its Medicare Advantage and Medicare Supplement plans through the first half of the year.

Part of the explanation for its growth is that UnitedHealth benefits from being the biggest, exactly as the Cigna CEO was told never to say out loud. It means it can have the best-known brand. It means having the clout to wring costs from a broad network of health care providers.

It means having the budgets to invest in the kind of technology and back-office capabilities that help its Medicare members stay healthier, from being able to prod members to choose healthier nutrition choices to helping seniors who call in for other reasons to also get their doctors’ appointments made.

UnitedHealth CEO Stephen Hemsley struck an expansively optimistic tone when updating shareholders on the quarterly conference call earlier this month. He reminded listeners of his opinion that his company, which just reported a quarter with 28 percent revenue growth, is “in the early stages of a unique era … looking ahead toward what may be the best and most important decade of performance.”

It’ll be interesting to see how the market for health insurance unfolds over the next few years now that the consolidation era seems to be over. There may be a day coming when UnitedHealth’s position in the market causes the same regulators to try to remember just how UnitedHealth got that big.