Statutory Accounting and Regulatory Requirements - Insurance Subsidiaries (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Insurance [Abstract]
Capital and surplus	$ 382,062	$ 394,016	$ 371,365
Net income	61,628	79,858	71,937
Net realized losses on investments	$ (647)	$ (504)	$ (3,131)