Asset Fair Value Measurements Using Significant Unobservable Inputs (Level 3) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 31	$ 36
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)
Included in other comprehensive income		2
Purchases	16
Purchases, sales, issuances, and settlements		(1)
Sales	(1)
Transfers into Level 3	11	4
Transfers out of Level 3	(32)	(10)
Ending balance	25	31
Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	20	27
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)
Included in other comprehensive income		2
Purchases	16
Purchases, sales, issuances, and settlements		(3)
Transfers into Level 3	10	4
Transfers out of Level 3	(28)	(10)
Ending balance	18	20
US States and Political Subdivisions Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	4	4
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)
Sales	(1)
Ending balance	3	4
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	5	5
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)
Transfers into Level 3	1
Transfers out of Level 3	(2)
Ending balance	4	5
Taxable Fixed Maturities | Separate Accounts Asset
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	2
Total gains or losses (realized/unrealized):
Included in earnings (or changes in net assets)
Purchases, sales, issuances, and settlements		2
Transfers out of Level 3	(2)
Ending balance		$ 2