I think we can fairly say that the idle power is roughly 150w. But, that is a lot of amp, so we would expect some considerable power at idle especially since at low power it is running Class A. But, you do get some benefit from that. As with everything there are trade offs.
If the idle power seems too high, then simply don't let is sit idle any more than you have to.
I'm curious why you are concerned about this?
Steve/bluewizard