Detachment for me was more of an emotional thing than anything. I didn't let myself get angry, surprised or sad that he was drinking. Along with detachment, I set boundaries for myself, such as "I will not buy him booze or give him money to do so", and "I will not be in his presence when he drinks". The result of this was that we spent very little time together, since he drank every night and all week-end. He started staying over a friends' houses, not coming home for days on end. In the end, we were like roommates more than anything else.
Leaving was the next logical step.