Condensed Financial Information - Statement of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Revenues:
Net investment income	$ 13,616	$ 13,597	$ 15,777	$ 11,258	$ 12,428	$ 16,259	$ 17,429	$ 17,384	$ 54,248	$ 63,500	$ 71,662
Total revenues	244,629	220,409	215,902	196,976	211,323	191,586	194,253	169,223	877,916	766,385	776,975
Expenses:
Interest expense	2,051	2,049	2,049	2,049	2,048	2,047	2,047	2,046	8,198	8,188	8,178
Other (income) expense	599	(789)	(387)	(911)	(586)	921	(573)	(991)	(1,488)	(1,229)	(5,143)
Total expenses	203,342	202,269	194,778	185,791	199,113	173,212	179,717	181,609	786,180	733,651	678,146
Income (loss) before income taxes	41,287	18,140	21,124	11,185	12,210	18,374	14,536	(12,386)	91,736	32,734	98,829
Income tax expense (benefit)	13,243	5,225	6,225	3,281	2,122	4,476	5,032	(4,493)	27,974	7,137	29,251
Net income (loss)	28,044	12,915	14,899	7,904	10,088	13,898	9,504	(7,893)	63,762	25,597	69,578
Parent Company
Revenues:
Net investment income									341	595	630
Dividends received from wholly-owned subsidiaries									15,000	45,000	40,000
Total revenues									15,341	45,595	40,630
Expenses:
Interest expense									8,198	8,188	8,178
Other (income) expense									1,749	2,969	634
Total expenses									9,947	11,157	8,812
Income (loss) before income taxes									5,394	34,438	31,818
Income tax expense (benefit)									(3,332)	(3,635)	(2,846)
Income before equity in undistributed net income of wholly owned subsidiaries									8,726	38,073	34,664
Equity in undistributed net income of wholly-owned subsidiaries									55,036	(12,476)	34,914
Net income (loss)									$ 63,762	$ 25,597	$ 69,578