Condensed Financial Information - Statement of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenues:
Net investment income	$ 16,071	$ 15,839	$ 15,648	$ 16,610	$ 14,254	$ 14,094	$ 14,246	$ 13,657	$ 64,168	$ 56,251	$ 54,248
Total revenues	251,617	269,260	249,540	253,114	248,729	224,970	222,490	221,375	1,023,531	917,564	877,916
Expenses:
Call premium on Senior Notes					17,895					17,895
Interest expense	3,854	3,388	4,319	3,852	4,351	2,053	2,052	2,051	15,413	10,507	8,198
Other (income) expense	(586)	(1,336)	1,665	(10,399)	665	210	915	(618)	(10,656)	1,172	(1,488)
Total expenses	223,519	223,003	224,681	211,792	228,320	193,880	202,269	200,822	882,995	825,291	786,180
Income (loss) before income tax benefit	28,098	46,257	24,859	41,322	20,409	31,090	20,221	20,553	140,536	92,273	91,736
Income tax benefit	8,823	15,032	7,998	13,354	6,076	9,804	6,284	6,643	45,207	28,807	27,974
Net income	19,275	31,225	16,861	27,968	14,333	21,286	13,937	13,910	95,329	63,466	63,762
Parent Company
Revenues:
Net investment income									76	13	341
Dividends received from wholly-owned subsidiaries											15,000
Total revenues									76	13	15,341
Expenses:
Call premium on Senior Notes										17,895
Interest expense									15,413	10,507	8,198
Other (income) expense										2	1,749
Total expenses									15,413	28,404	9,947
Income (loss) before income tax benefit									(15,337)	(28,391)	5,394
Income tax benefit									(5,287)	(9,886)	(3,332)
Income (loss) before equity in undistributed net income of wholly owned subsidiaries									(10,050)	(18,505)	8,726
Equity in undistributed net income of wholly-owned subsidiaries									105,379	81,971	55,036
Net income									$ 95,329	$ 63,466	$ 63,762