Losses and Loss Expenses Payable - Schedule of Activity in Liability for Losses and Loss Expenses (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Insurance [Abstract]
Losses and loss expenses payable, affiliates	$ 630.9	$ 532.4	$ 494.3