Fair Value Measurements - Change in Level 3 Assets (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 6	$ 53	$ 78	$ 55
Total gains or losses (realized/unrealized):
Included in net income	0	0	0	0
Included in other comprehensive income	0	1	0	0
Purchases	0	0	5	22
Sales	0	(1)	0	(3)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	(11)	(77)	(32)
Ending balance	6	42	6	42
Corporate Fixed Maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1	52	78	51
Total gains or losses (realized/unrealized):
Included in net income	0	0	0	0
Included in other comprehensive income	0	1	0	1
Purchases	0	0	0	22
Sales	0	(1)	0	(1)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	(10)	(77)	(31)
Ending balance	1	42	1	42
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	(1)	0	(1)
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	5	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	0	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	0	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	0	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