Fair Value Measurements - Change in Level 3 Assets (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 21	$ 11	$ 20	$ 4
Total gains or losses (realized/unrealized):
Included in net income	0	0	0	0
Included in other comprehensive income	1	0	1	0
Purchases	22	0	22	0
Sales	0	0	0	0
Transfers into Level 3	0	19	1	31
Transfers out of Level 3	(1)	0	(1)	(5)
Ending balance	43	30	43	30
Corporate Fixed Maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	18	9	18	2
Total gains or losses (realized/unrealized):
Included in net income	0	0	0	0
Included in other comprehensive income	1	0	1	0
Purchases	22	0	22	0
Sales	0	0	0	0
Transfers into Level 3	0	19	0	26
Transfers out of Level 3	0	0	0	0
Ending balance	41	28	41	28
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0	0	0
Total gains or losses (realized/unrealized):
Included in net income	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Transfers into Level 3	0	0	0	5
Transfers out of Level 3	0	0	0	(5)
Ending balance	0	0	0	0
States, Municipalities And Political Subdivisions Fixed Maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1	0	0	0
Total gains or losses (realized/unrealized):
Included in net income	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Transfers into Level 3	0	0	1	0
Transfers out of Level 3	(1)	0	(1)	0
Ending balance	0	0	0	0
Preferred Equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2	2	2	2
Total gains or losses (realized/unrealized):
Included in net income	0	0	0	0
Included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	$ 2	$ 2	$ 2	$ 2