Investments - Effect of Trading Activities by Type of Instrument and by Line Item in Condensed Consolidated Statements of Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Net Investment Income [Line Items]
Total realized gains (losses) on trading portfolio	$ (1)	$ (1,705)	$ (16,038)	$ (9,154)
Total change in unrealized gains (losses) on trading portfolio	23	(5,788)	7,897	3,399
Net gains (losses) recognized on trading portfolio		(7,493)	(8,163)	(5,755)
Fixed Maturities [Member]
Net Investment Income [Line Items]
Total realized gains (losses) on trading portfolio
Total change in unrealized gains (losses) on trading portfolio		100	13	137
Equity Securities [Member]
Net Investment Income [Line Items]
Total realized gains (losses) on trading portfolio		(1,836)	(15,969)	(9,429)
Total change in unrealized gains (losses) on trading portfolio		(5,817)	7,758	3,172
Derivatives (Non-Hedging Instruments) [Member]
Net Investment Income [Line Items]
Total realized gains (losses) on trading portfolio		131	(68)	275
Total change in unrealized gains (losses) on trading portfolio		(30)	89	117
Other Investments [Member]
Net Investment Income [Line Items]
Total change in unrealized gains (losses) on trading portfolio		$ (41)	$ 14	$ (27)