Realized Gains and Losses Excluding Net Other-Than-Temporary Impairment Losses Recognized in 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
Gain (Loss) on Investments [Line Items]
Net realized gains (losses)	$ 7,141	$ 3,238	$ 3,006	$ (1,389)	$ 19,665	$ 4,521	$ 11,020	$ 6,113	$ 11,996	$ 41,319	$ 9,217
Fixed maturities
Gain (Loss) on Investments [Line Items]
Gains									11,678	42,932	18,312
Losses									(7,044)	(3,239)	(9,676)
Net realized gains (losses)									4,634	39,693	8,636
Equity Securities
Gain (Loss) on Investments [Line Items]
Gains									9,319	1,867	2,110
Losses									(1,957)	(241)	(1,529)
Net realized gains (losses)									$ 7,362	$ 1,626	$ 581