BOSTON — Leaders of Boston's bid for the 2024 Summer Olympics released details Thursday about insurance proposals they say will provide unprecedented coverage for Massachusetts taxpayers.The Boston 2024 bid committee released a 13-page document that described at least eight types of insurance it promises to purchase to minimize the risk to taxpayers in the event of unanticipated costs.The private organization has estimated the insurance will cost about $128 million, which it said it would pay for fully.Things the insurance policies would cover include: