Fair Value Measurements - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Level 3 securities purchased	$ 16
Securities transferred into Level 3	11	4
Securities transferred out of Level 3	32	10
Purchases, sales, issuances, and settlements		(1)
Corporate | Debt Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of securities purchased	3
Level 3 securities purchased	16
Number of securities transferred into Level 3	3
Securities transferred into Level 3	10
Number of securities transferred out of Level 3	6
Securities transferred out of Level 3	28
Purchases, sales, issuances, and settlements	$ (2)
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of Level 3 assets measured at fair value	1.00%	1.00%