Heart attack apparently. Surprising, but not shocking, given how he was living the last 2 decades. He just seemed like one of those hard living guys that would be here forever because he had an iron constitution.
Tremendous body of work, and testament to his ability as an actor is how much people in Hollywood looked out for him even though he developed an unreliable reputation. His son committed suicide a few years ago, and that seems to destroy him a bit.
RIP Mr Blonde, I hope you find peace.