Statutory Accounting and Regulatory Requirements - Insurance Subsidiaries (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Insurance [Abstract]
Capital and surplus	$ 383,575	$ 382,062	$ 394,016
Net income	72,979	61,628	79,858
Net realized losses on investments	$ (1,536)	$ (647)	$ (504)