Condensed Financial Information - Statement of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Revenues:
Net investment income	$ 12,428	$ 16,259	$ 17,429	$ 17,384	$ 17,998	$ 17,839	$ 17,853	$ 17,972	$ 63,500	$ 71,662	$ 75,512
Total revenues	211,323	191,586	194,253	169,223	205,851	192,691	189,290	189,143	766,385	776,975	762,880
Expenses:
Interest expense	2,048	2,047	2,047	2,046	2,045	2,045	2,044	2,044	8,188	8,178	8,455
Other (income) expense	(586)	921	(573)	(991)	(2,205)	(2,767)	899	(1,070)	(1,229)	(5,143)	(6,665)
Total expenses	199,113	173,212	179,717	181,609	180,921	169,375	162,097	165,753	733,651	678,146	676,032
Income before income tax benefit	12,210	18,374	14,536	(12,386)	24,930	23,316	27,193	23,390	32,734	98,829	86,848
Income tax expense (benefit)	(2,122)	(4,476)	(5,032)	4,493	(7,592)	(7,091)	(8,223)	(6,345)	(7,137)	(29,251)	(23,690)
Net income (loss)	10,088	13,898	9,504	(7,893)	17,338	16,225	18,970	17,045	25,597	69,578	63,158
Parent Company
Revenues:
Net investment income									595	630	583
Dividends received from wholly-owned subsidiaries									45,000	40,000	25,000
Total revenues									45,595	40,630	25,583
Expenses:
Interest expense									8,188	8,178	8,455
Other (income) expense									2,969	634	(1,482)
Total expenses									11,157	8,812	6,973
Income before income tax benefit									34,438	31,818	18,610
Income tax expense (benefit)									(3,635)	(2,846)	(2,174)
Income before equity in undistributed net income of wholly owned subsidiaries									38,073	34,664	20,784
Equity in undistributed net income of wholly-owned subsidiaries									(12,476)	34,914	42,374
Net income (loss)									$ 25,597	$ 69,578	$ 63,158