Condensed Financial Information - Statement of Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenues:
Net investment income	$ 20,107	$ 19,875	$ 19,875	$ 19,594	$ 18,499	$ 17,371	$ 16,595	$ 16,253	$ 79,451	$ 68,718	$ 64,168
Total revenues	310,040	305,288	294,107	288,098	272,111	268,178	258,477	260,223	1,197,533	1,058,989	1,023,531
Expenses:
Interest expense	3,860	3,859	3,858	3,858	3,857	3,856	3,856	3,855	15,435	15,424	15,413
Other (income) expense	(1,905)	183	(4,430)	(2,549)	(4,147)	2,518	4,362	(2,242)	(8,701)	491	(10,656)
Total expenses	278,583	275,400	270,864	255,177	248,296	241,254	230,098	221,867	1,080,024	941,515	882,995
Income before income taxes	31,457	29,888	23,243	32,921	23,815	26,924	28,379	38,356	117,509	117,474	140,536
Income tax benefit	9,866	7,875	7,053	9,989	6,072	8,723	9,195	12,427	34,783	36,417	45,207
Net income	$ 21,591	$ 22,013	$ 16,190	$ 22,932	$ 17,743	$ 18,201	$ 19,184	$ 25,929	82,726	81,057	95,329
Parent Company
Revenues:
Net investment income									53	147	76
Total revenues									53	147	76
Expenses:
Interest expense									15,435	15,424	15,413
Other (income) expense									22
Total expenses									15,457	15,424	15,413
Income before income taxes									(15,404)	(15,277)	(15,337)
Income tax benefit									(8,009)	(5,472)	(5,287)
Income (loss) before equity in undistributed net income of wholly owned subsidiaries									(7,395)	(9,805)	(10,050)
Equity in undistributed net income of wholly-owned subsidiaries									$ 90,121	$ 90,862	$ 105,379