Condensed Statement of Earnings (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									$ 108,910	$ 125,012	$ 101,949
Net realized capital gains									127,141	97,374	320,389
Other than temporary impairment losses									(3,607)	(12,356)	(85,916)
Other income									1,754	7,188	2,955
Total revenues	282,000	232,500	219,200	248,100	232,900	250,200	250,600	251,700	981,837	985,352	1,184,392
Costs and Expenses:
Interest expense									17,426	4,698	633
Corporate administration									41,007	28,854	26,938
Total costs and expenses									791,000	707,981	789,012
Earnings before income taxes									190,837	277,371	395,380
Income taxes									47,586	78,869	124,381
Net earnings	37,700	19,200	15,100	71,300	37,400	36,600	66,300	58,200	143,251	198,502	270,999
Parent Company
Revenues:
Net investment income									13,240	2,642	8,341
Net realized capital gains									46,844	4,466	200,626
Other than temporary impairment losses
Other income									1,031	1,492
Total revenues									61,115	8,600	208,967
Costs and Expenses:
Interest expense									17,287	5,004	633
Corporate administration									40,962	28,592	27,022
Total costs and expenses									58,249	33,596	27,655
Operating profit (losses)									2,866	(24,996)	181,312
Equity in earnings of consolidated subsidiaries									187,971	302,367	214,068
Earnings before income taxes									190,837	277,371	395,380
Income taxes									47,586	78,869	124,381
Net earnings									$ 143,251	$ 198,502	$ 270,999