Fair Value - Reconciliations of Assets and Liabilities Measured at Fair Value on Recurring Basis Using Significant Unobservable Inputs (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Corporate Bonds and Other [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 100	$ 121	$ 98	$ 130
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income (Loss)			(1)
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI	(1)		(1)	1
Purchases	2		2	5
Sales	(5)		(5)	(1)
Settlements	(2)	(11)	(4)	(25)
Transfers into Level 3			5
Transfers out of Level 3		(10)		(10)
Ending balance	94	100	94	100
States, Municipalities and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	1	1	1	1
Ending balance	1	1	1	1
Total Asset-Backed [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	279	256	335	199
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income (Loss)		1	7	1
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI	(1)	1	(6)	3
Purchases	41	13	71	51
Sales			(72)
Settlements	(6)	(7)	(12)	(13)
Transfers into Level 3	13	24	13	52
Transfers out of Level 3	(53)	(70)	(63)	(75)
Ending balance	273	218	273	218
Total Fixed Maturities Available-for-Sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	380	378	434	330
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income (Loss)		1	6	1
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI	(2)	1	(7)	4
Purchases	43	13	73	56
Sales	(5)		(77)	(1)
Settlements	(8)	(18)	(16)	(38)
Transfers into Level 3	13	24	18	52
Transfers out of Level 3	(53)	(80)	(63)	(85)
Ending balance	368	319	368	319
Fixed Maturities Trading [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	7	5	4	6
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income (Loss)			3	(1)
Ending balance	7	5	7	5
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities		(1)	3	(1)
Fixed Maturity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	387	383	438	336
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income (Loss)		1	9
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI	(2)	1	(7)	4
Purchases	43	13	73	56
Sales	(5)		(77)	(1)
Settlements	(8)	(18)	(16)	(38)
Transfers into Level 3	13	24	18	52
Transfers out of Level 3	(53)	(80)	(63)	(85)
Ending balance	375	324	375	324
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities		(1)	3	(1)
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance	20	20	22	20
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income (Loss)	(1)		(3)
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI		1		2
Purchases				1
Sales	(1)	(1)	(1)	(3)
Ending balance	18	20	18	20
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities	$ (1)		$ (3)
Equity Securities Available-for-Sale [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		19		19
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI		1		2
Purchases				1
Sales		(1)		(3)
Ending balance		19		19
Equity Securities Trading [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		1		1
Ending balance		1		1
Life Settlement Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning balance		46		58
Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income (Loss)				6
Sales		(45)		(58)
Settlements				(5)
Ending balance		1		1
Derivative Instruments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative financial instruments, net, Beginning balance				0
Derivative financial instruments, net, Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in Net Income (Loss)				1
Derivative financial instruments, net, Net Realized Gains (Losses) and Net Change in Unrealized Gains (Losses), Included in OCI				0
Derivative financial instruments, net, Purchases				0
Derivative financial instruments, net, Sales				(1)
Derivative financial instruments, net, Settlements				0
Derivative financial instruments, net, Transfers into Level 3				0
Derivative financial instruments, net, Transfers out of Level 3				0
Derivative financial instruments, net, Ending balance		$ 0		0
Unrealized Gains (Losses) Recognized in Net Income (Loss) on Level 3 Assets and Liabilities				$ 0