Investments (Detail) - Credit loss impairments on fixed income securities (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Balance at January 1,	$ 333,379	$ 426,372
Credit loss impairment recognized in the current period on securities not previously impaired	1,835	4,573
Credit loss impairments previously recognized on securities which matured, paid down, prepaid or were sold during the period	(18,940)	(125,711)
Credit loss impairments previously recognized on securities impaired to fair value during the period	(16,384)	0
Additional credit loss impairments recognized in the current period on securities previously impaired	14,930	25,459
Accretion of credit loss impairments previously recognized due to an increase in cash flows expected to be collected	(15)	(523)
Balance at March 31,	$ 314,805	$ 330,170