Statutory Accounting and Regulatory Requirements - Insurance Subsidiaries (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Insurance [Abstract]
Capital and surplus	$ 365,524	$ 359,952	$ 383,575
Net income	82,390	92,301	72,979
Net realized gains (losses) on investments	$ 1,075	$ (80)	$ (1,536)