Fair Value Measurements - Change in Level 3 Assets (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 5	$ 25
Total gains or losses (realized/unrealized):
Included in earnings	0	0
Included in other comprehensive income	0	5
Purchases	1	1
Sales	0	(4)
Transfers into Level 3	0	1
Transfers out of Level 3	0	(3)
Ending balance	6	25
Corporate Fixed Maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	3	18
Total gains or losses (realized/unrealized):
Included in earnings	0	0
Included in other comprehensive income	0	3
Purchases	0	0
Sales	0	(3)
Transfers into Level 3	0	1
Transfers out of Level 3	0	(3)
Ending balance	3	16
States, Municipalities And Political Subdivisions Fixed Maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1	3
Total gains or losses (realized/unrealized):
Included in earnings	0	0
Included in other comprehensive income	0	0
Purchases	0	0
Sales	0	(1)
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending balance	1	2
Preferred Equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1	4
Total gains or losses (realized/unrealized):
Included in earnings	0	0
Included in other comprehensive income	0	2
Purchases	1	1
Sales	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Ending balance	$ 2	$ 7