Fair Value Measurements - Change in Level 3 Assets (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 1	$ 54	$ 78	$ 55
Total gains or losses (realized/unrealized):
Included in net income	0	0	0	0
Included in other comprehensive income	0	0	0	(1)
Purchases	0	17	5	22
Sales	0	(2)	0	(2)
Transfers into Level 3	5	0	0	0
Transfers out of Level 3	0	(16)	(77)	(21)
Ending balance	6	53	6	53
Corporate Fixed Maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1	51	78	51
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	17	0	22
Sales	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	(16)	(77)	(21)
Ending balance	1	52	1	52
Taxable Fixed Maturities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	1	0	1
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	0	1	0	1
States, Municipalities And Political Subdivisions Fixed Maturities
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	5	0
Sales	0	0	0	0
Transfers into Level 3	5	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	5	0	5	0
Preferred Equities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	2	0	3
Total gains or losses (realized/unrealized):
Included in net income	0	0	0	0
Included in other comprehensive income	0	0	0	(1)
Purchases	0	0	0	0
Sales	0	(2)	0	(2)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Ending balance	$ 0	$ 0	$ 0	$ 0