Selected Quarterly Financial Data (Unaudited) - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Effect of federal tax law change	$ 12,500		$ 12,509
Unfavorable loss development included in income before income taxes		$ 25,800	$ 220,785	$ 309,002	$ 219,031