Investments (Detail) - Credit loss impairments on fixed income securities (USD $) In Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Opening balance	$ 315,844	$ 423,041	$ 426,372	$ 537,121
Credit loss impairment recognized in the current period on securities not previously impaired	10,478	5,342	26,384	24,800
Credit loss impairments previously recognized on securities that matured, paid down, prepaid or were sold during the period	(16,448)	(36,046)	(180,475)	(74,024)
Credit loss impairments previously recognized on securities impaired to fair value during the period				(130,891)
Additional credit loss impairments recognized in the current period on securities previously impaired	35,574	26,280	74,372	76,498
Accretion of credit loss impairments previously recognized due to an increase in cash flows expected to be collected	(2,173)	(6,361)	(3,378)	(21,248)
Balance, September 30	$ 343,275	$ 412,256	$ 343,275	$ 412,256