Condensed Financial Information - Statement of Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenues:
Net investment income	$ 18,499	$ 17,371	$ 16,595	$ 16,253	$ 16,071	$ 15,839	$ 15,648	$ 16,610	$ 68,718	$ 64,168	$ 56,251
Total revenues	272,111	268,178	258,477	260,223	251,617	269,260	249,540	253,114	1,058,989	1,023,531	917,564
Expenses:
Call premium on Senior notes											17,895
Interest expense	3,857	3,856	3,856	3,855	3,854	3,388	4,319	3,852	15,424	15,413	10,507
Other (income) expense	(4,147)	2,518	4,362	(2,242)	(586)	(1,336)	1,665	(10,399)	491	(10,656)	1,172
Total expenses	248,296	241,254	230,098	221,867	223,519	223,003	224,681	211,792	941,515	882,995	825,291
Income before income taxes	23,815	26,924	28,379	38,356	28,098	46,257	24,859	41,322	117,474	140,536	92,273
Income tax (benefit) expense	6,072	8,723	9,195	12,427	8,823	15,032	7,998	13,354	36,417	45,207	28,807
Net income	$ 17,743	$ 18,201	$ 19,184	$ 25,929	$ 19,275	$ 31,225	$ 16,861	$ 27,968	81,057	95,329	63,466
Parent Company
Revenues:
Net investment income									147	76	13
Total revenues									147	76	13
Expenses:
Call premium on Senior notes											17,895
Interest expense									15,424	15,413	10,507
Other (income) expense											2
Total expenses									15,424	15,413	28,404
Income before income taxes									(15,277)	(15,337)	(28,391)
Income tax (benefit) expense									(5,472)	(5,287)	(9,886)
Income (loss) before equity in undistributed net income of wholly owned subsidiaries									(9,805)	(10,050)	(18,505)
Equity in undistributed net income of wholly-owned subsidiaries									90,862	105,379	81,971
Net income									$ 81,057	$ 95,329	$ 63,466