Basically, I have about 7 or 8 characters (excluding the antagonist) who each have important roles in the story. My problem is that my characters tend to develop themselves, so to speak. I have no problem with this - in fact, I find it rather amusing - but it leads to an increased chance of them going out of character later. The way to prevent this is, obviously, to write up a bio - the character's personality, back story (to some extent), and some other basic traits.
The question is, how detailed should I be? I don't want to be overly basic - again, that leads to out of character moments (and plot holes) - but I also don't want to go into extreme detail on something only I am likely to see. I want to at least include some important events that helped shape who the character is now. I've finished one, and it's almost over the top in detail. I'd rather not do that 6 more times.
I'd like some advice on what I should and should not include in a character bio, as well as the level of detail I should go into.