Selected Quarterly Financial Data (Unaudited) - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Jun. 30, 2016	Sep. 30, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Unfavorable loss development included in income before income taxes	$ 25,800		$ 309,002	$ 219,031	$ 161,302
Litigation settlement, amount		$ 3,400		$ 3,200