The most detailed (public available) document seems to be https://github.com/input-output-hk/cardano-sl/blob/fe6653822a6bc0aa007d6255a8511a562aad222e/docs/delegation_design_spec.md
There is a way more readable summary on Lars Brünjes presentation at Summary: PoS Delegation & Incentives (Lars Brunjes)
and also a just this week published statement at Cardano Netherlands Meetup
It was this questions rised at Rotterdam who let me look up for past statements like this one here.