Investments (Detail) - Credit loss impairments on fixed income securities (USD $) In Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Opening balance	$ 330,170	$ 551,748	$ 426,372	$ 537,121
Credit loss impairment recognized in the current period on securities not previously impaired	11,333	9,384	15,906	19,458
Credit loss impairments previously recognized on securities that matured, paid down, prepaid or were sold during the period	(38,316)	(22,003)	(164,027)	(37,978)
Credit loss impairments previously recognized on securities impaired to fair value during the period		(130,891)		(130,891)
Additional credit loss impairments recognized in the current period on securities previously impaired	13,339	22,591	38,798	50,218
Accretion of credit loss impairments previously recognized due to an increase in cash flows expected to be collected	(682)	(7,788)	(1,205)	(14,887)
Balance, June 30	$ 315,844	$ 423,041	$ 315,844	$ 423,041