System Idle Process means that the CPU isnt being used. Which show me that the game was not being responsive to the system. Just shutting down Steam usually doesnt work cause it has other resources still running. In the task manger there is still the Steam process even if you close it. Have to really force it to close.
Why you had to shut down the PSU to get this to stop is not a good sign. A simple restart should have taken care of it. I would start doing some stress tests to find out if any of your hardware is dying.