Statutory Accounting and Regulatory Requirements - Insurance Subsidiaries (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Insurance [Abstract]
Capital and surplus	$ 394,016	$ 371,365	$ 377,742
Net income	79,858	71,937	50,205
Net realized gains (losses) on investments	$ (504)	$ (3,131)	$ 693