QI was stunned to find out that someone I work closely with took an idea of mine and passed it off as his own. I'm not sure what to do -- call him on it or let it go?
AIdea theft doesn't need to be tolerated, but careful steps are necessary to ensure you maintain your own professionalism when dealing with it.
The inner game
This is an emotional issue, and you likely have a range of feelings from volatile ones like anger or passive ones like helplessness and betrayal. Either way, get your feelings under control so that you neither overreact nor give up.
Now, consider your past experience with this co-worker. Does being "stunned" imply that this behavior seems out of character, or are you just surprised that he was this blatant? If there's a pattern, even a subtle one, you'll need to set up more safeguards in the future.
Reflect on your interactions regarding this idea. Could he see it as building on a small nugget of a concept and transforming it? Or did he take credit for a full-fledged idea? Also consider how big a deal it was -- was it a minor insight or a groundbreaking new direction? Do you have documentation or witnesses that establish your contribution, if needed?
Finally, assess the credibility of your source -- do the motives or quality of information of the individual who told you about the situation stand up to scrutiny?
These lines of inquiry are not intended to suggest that you weren't shortchanged on credit for a good idea. However, accusing someone of idea theft, even obliquely, is serious, and you need to be on solid ground if you want to talk to him about it, or raise it with your boss or others in authority.
The outer game
Starting from the most benign interpretation, make a plan to address the situation if you think it was an innocent mistake, perhaps by an inexperienced employee. A hallway conversation without others around may be a good option. "By the way ... " can make your point without escalation.
If you determine that he intentionally took credit for an important idea, stronger action may be appropriate. Meet with your boss to think through next steps, including a talk with your co-worker. Also plan what you'll do if he denies that it was your idea or doesn't have a satisfactory response for you. If necessary, your boss or other leadership may need to be involved.
Keep your conversation direct, nonemotional and nonaccusatory: "Remember that idea I discussed with you? I've heard that you've since presented it to the executive team, and took full credit for it. This was surprising to me, and I'd like to hear your perspective." Calling him on this perceived misbehavior may be enough to stop this in the future. If not, move on to the next steps in your plan.
Look at the possibility that you're overly reticent with your ideas. Holding back on sharing powerful ideas invites others to move forward with them, and holds you back professionally
The last word
Stand up for yourself in a clear but professional way in order to protect yourself from idea theft.