Change in Level 3 Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 13	$ 26	$ 25	$ 31
Total gains or losses (realized/unrealized):
Included in earnings
Included in other comprehensive income		(1)	5
Purchases		8	1	15
Sales	(1)	(1)	(6)	(1)
Transfers into Level 3		7	1	8
Transfers out of Level 3		(8)	(14)	(22)
Ending balance	12	31	12	31
Corporate Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	4	15	18	20
Total gains or losses (realized/unrealized):
Included in earnings
Included in other comprehensive income			3
Purchases		8		15
Sales			(4)
Transfers into Level 3		7	1	7
Transfers out of Level 3		(8)	(14)	(20)
Ending balance	4	22	4	22
US States and Political Subdivisions Debt Securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	2	4	3	4
Total gains or losses (realized/unrealized):
Included in earnings
Sales	(1)	(1)	(2)	(1)
Ending balance	1	3	1	3
Preferred Stock
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	7	7	4	5
Total gains or losses (realized/unrealized):
Included in earnings
Included in other comprehensive income		(1)	2
Purchases			1
Transfers into Level 3				1
Ending balance	7	6	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)