Change in Level 3 Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 25	$ 31
Total gains or losses (realized/unrealized):
Included in earnings
Included in other comprehensive income	5
Purchases	1
Sales	(4)
Transfers into Level 3	1	1
Transfers out of Level 3	(3)	(11)
Ending balance	25	21
Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	18	20
Total gains or losses (realized/unrealized):
Included in earnings
Included in other comprehensive income	3
Sales	(3)
Transfers into Level 3	1
Transfers out of Level 3	(3)	(9)
Ending balance	16	11
US States and Political Subdivisions Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	3	4
Total gains or losses (realized/unrealized):
Included in earnings
Sales	(1)
Ending balance	2	4
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	4	5
Total gains or losses (realized/unrealized):
Included in earnings
Included in other comprehensive income	2
Purchases	1
Transfers into Level 3		1
Ending balance	7	6
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
Transfers out of Level 3		$ (2)