Fair Value Measurements - Fair Value by Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Fair Value Measurement [Domain]
Assets, Fair Value Disclosure [Abstract]
Separate Account Assets	$ 105,063	$ 113,302
Available-for-sale Securities, Debt Securities	21,781	22,799
Fair Value, Option, Fixed Maturity Securities	29	32
Trading Securities, Equity		12
Marketable Securities, Equity Securities	124
Available-for-sale Securities, Equity Securities	0	154
Derivative Asset		135
Other Short-term Investments	1,493	1,094
Reinsurance Recoverables, Including Reinsurance Premium Paid	20,701	20,785
Separate Account Assets	108,393	115,834
Assets, Fair Value Disclosure	128,680	137,619
Liabilities, Fair Value Disclosure [Abstract]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	515	663
Portion at Other than Fair Value Measurement [Member]
Assets, Fair Value Disclosure [Abstract]
Separate Account Assets	2,800	2,500
Estimate of Fair Value Measurement [Member]
Assets, Fair Value Disclosure [Abstract]
Separate Account Assets	509	900
Liability [Member]
Liabilities, Fair Value Disclosure [Abstract]
Obligations, Fair Value Disclosure	(53)	(75)
Guaranteed Minimum Withdrawal Benefit [Member]
Liabilities, Fair Value Disclosure [Abstract]
Obligations, Fair Value Disclosure	(53)	(75)
Credit Risk Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	4	1
Interest Rate Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	(2)	47
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	327	403
US GMWB Hedging Instruments [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	57	69
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	10	(2)
Macro Hedge Program [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	19	19
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	16	4
Derivative Financial Instruments, Assets [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	78
GMWB Reinsurance [Member]
Assets, Fair Value Disclosure [Abstract]
Reinsurance Recoverables, Including Reinsurance Premium Paid	31	36
Coinsurance and Modified Coinsurance Reinsurance Contracts [Member]
Assets, Fair Value Disclosure [Abstract]
Reinsurance Recoverables, Including Reinsurance Premium Paid	81	55
Foreign Exchange Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset		(1)
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	161	187
Derivative Financial Instruments, Liabilities [Member]
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	(462)	(588)
ABS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	829	819
CDOs
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	780	888
CMBS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	1,987	2,084
Corporate
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	13,424	14,038
Foreign Government Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	415	407
US States and Political Subdivisions Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	1,238	1,266
RMBS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	1,352	1,427
U.S. Treasuries
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	1,756	1,870
Fair Value, Inputs, Level 1 [Member] | Fair Value Measurement [Domain]
Assets, Fair Value Disclosure [Abstract]
Separate Account Assets	70,204
Fair Value, Inputs, Level 1 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	216	284
Fair Value, Option, Fixed Maturity Securities	0	0
Trading Securities, Equity		12
Marketable Securities, Equity Securities	33
Available-for-sale Securities, Equity Securities		61
Other Short-term Investments	677	807
Separate Account Assets		73,538
Assets, Fair Value Disclosure	71,130	74,702
Liabilities, Fair Value Disclosure [Abstract]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 1 [Member] | Liability [Member]
Liabilities, Fair Value Disclosure [Abstract]
Obligations, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 1 [Member] | Guaranteed Minimum Withdrawal Benefit [Member]
Liabilities, Fair Value Disclosure [Abstract]
Obligations, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 1 [Member] | Credit Risk Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	0	0
Fair Value, Inputs, Level 1 [Member] | Interest Rate Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Fair Value, Inputs, Level 1 [Member] | US GMWB Hedging Instruments [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Fair Value, Inputs, Level 1 [Member] | Macro Hedge Program [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Fair Value, Inputs, Level 1 [Member] | GMWB Reinsurance [Member]
Assets, Fair Value Disclosure [Abstract]
Reinsurance Recoverables, Including Reinsurance Premium Paid	0	0
Fair Value, Inputs, Level 1 [Member] | Coinsurance and Modified Coinsurance Reinsurance Contracts [Member]
Assets, Fair Value Disclosure [Abstract]
Reinsurance Recoverables, Including Reinsurance Premium Paid	0	0
Fair Value, Inputs, Level 1 [Member] | Foreign Exchange Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Fair Value, Inputs, Level 1 [Member] | Derivative Financial Instruments, Liabilities [Member]
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Fair Value, Inputs, Level 1 [Member] | ABS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 1 [Member] | CDOs
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 1 [Member] | CMBS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 1 [Member] | Corporate
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 1 [Member] | Foreign Government Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 1 [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 1 [Member] | RMBS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Fair Value, Inputs, Level 1 [Member] | U.S. Treasuries
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	216	284
Fair Value, Inputs, Level 2 [Member] | Fair Value Measurement [Domain]
Assets, Fair Value Disclosure [Abstract]
Separate Account Assets	34,818
Fair Value, Inputs, Level 2 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	20,337	21,229
Fair Value, Option, Fixed Maturity Securities	29	32
Trading Securities, Equity		0
Marketable Securities, Equity Securities	49
Available-for-sale Securities, Equity Securities		47
Derivative Asset		82
Other Short-term Investments	816	287
Separate Account Assets		38,677
Assets, Fair Value Disclosure	56,153	60,409
Liabilities, Fair Value Disclosure [Abstract]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	444	563
Fair Value, Inputs, Level 2 [Member] | Liability [Member]
Liabilities, Fair Value Disclosure [Abstract]
Obligations, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 2 [Member] | Guaranteed Minimum Withdrawal Benefit [Member]
Liabilities, Fair Value Disclosure [Abstract]
Obligations, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 2 [Member] | Credit Risk Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	4	1
Fair Value, Inputs, Level 2 [Member] | Interest Rate Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	(2)	47
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	299	374
Fair Value, Inputs, Level 2 [Member] | US GMWB Hedging Instruments [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	21	35
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	14	(2)
Fair Value, Inputs, Level 2 [Member] | Macro Hedge Program [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	2	0
Fair Value, Inputs, Level 2 [Member] | Derivative Financial Instruments, Assets [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	23
Fair Value, Inputs, Level 2 [Member] | GMWB Reinsurance [Member]
Assets, Fair Value Disclosure [Abstract]
Reinsurance Recoverables, Including Reinsurance Premium Paid	0	0
Fair Value, Inputs, Level 2 [Member] | Coinsurance and Modified Coinsurance Reinsurance Contracts [Member]
Assets, Fair Value Disclosure [Abstract]
Reinsurance Recoverables, Including Reinsurance Premium Paid	81	55
Fair Value, Inputs, Level 2 [Member] | Foreign Exchange Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset		(1)
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	161	187
Fair Value, Inputs, Level 2 [Member] | Derivative Financial Instruments, Liabilities [Member]
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	(444)	(563)
Fair Value, Inputs, Level 2 [Member] | ABS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	820	806
Fair Value, Inputs, Level 2 [Member] | CDOs
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	708	815
Fair Value, Inputs, Level 2 [Member] | CMBS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	1,966	2,058
Fair Value, Inputs, Level 2 [Member] | Corporate
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	12,977	13,595
Fair Value, Inputs, Level 2 [Member] | Foreign Government Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	414	406
Fair Value, Inputs, Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	1,201	1,228
Fair Value, Inputs, Level 2 [Member] | RMBS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	711	735
Fair Value, Inputs, Level 2 [Member] | U.S. Treasuries
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	1,540	1,586
Fair Value, Inputs, Level 3 [Member] | Fair Value Measurement [Domain]
Assets, Fair Value Disclosure [Abstract]
Separate Account Assets	41
Fair Value, Inputs, Level 3 [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	1,228	1,286
Fair Value, Option, Fixed Maturity Securities	0	0
Trading Securities, Equity		0
Marketable Securities, Equity Securities	42
Available-for-sale Securities, Equity Securities		46
Derivative Asset		53
Other Short-term Investments	0	0
Separate Account Assets		185
Assets, Fair Value Disclosure	1,397	1,606
Liabilities, Fair Value Disclosure [Abstract]
Financial and Nonfinancial Liabilities, Fair Value Disclosure	71	100
Fair Value, Inputs, Level 3 [Member] | Liability [Member]
Liabilities, Fair Value Disclosure [Abstract]
Obligations, Fair Value Disclosure	(53)	(75)
Fair Value, Inputs, Level 3 [Member] | Guaranteed Minimum Withdrawal Benefit [Member]
Liabilities, Fair Value Disclosure [Abstract]
Obligations, Fair Value Disclosure	(53)	(75)
Fair Value, Inputs, Level 3 [Member] | Credit Risk Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	0	0
Fair Value, Inputs, Level 3 [Member] | Interest Rate Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	0	0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	28	29
Fair Value, Inputs, Level 3 [Member] | US GMWB Hedging Instruments [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	36	34
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	(4)	0
Fair Value, Inputs, Level 3 [Member] | Macro Hedge Program [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	19	19
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	14	4
Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Assets [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset	55
Fair Value, Inputs, Level 3 [Member] | GMWB Reinsurance [Member]
Assets, Fair Value Disclosure [Abstract]
Reinsurance Recoverables, Including Reinsurance Premium Paid	31	36
Fair Value, Inputs, Level 3 [Member] | Coinsurance and Modified Coinsurance Reinsurance Contracts [Member]
Assets, Fair Value Disclosure [Abstract]
Reinsurance Recoverables, Including Reinsurance Premium Paid	0	0
Fair Value, Inputs, Level 3 [Member] | Foreign Exchange Contract [Member]
Assets, Fair Value Disclosure [Abstract]
Derivative Asset		0
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	0	0
Fair Value, Inputs, Level 3 [Member] | Derivative Financial Instruments, Liabilities [Member]
Liabilities, Fair Value Disclosure [Abstract]
Derivative Liability	(18)	(25)
Fair Value, Inputs, Level 3 [Member] | ABS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	9	13
Fair Value, Inputs, Level 3 [Member] | CDOs
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	72	73
Fair Value, Inputs, Level 3 [Member] | CMBS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	21	26
Fair Value, Inputs, Level 3 [Member] | Corporate
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	447	443
Fair Value, Inputs, Level 3 [Member] | Foreign Government Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	1	1
Fair Value, Inputs, Level 3 [Member] | US States and Political Subdivisions Debt Securities [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	37	38
Fair Value, Inputs, Level 3 [Member] | RMBS [Member]
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	641	692
Fair Value, Inputs, Level 3 [Member] | U.S. Treasuries
Assets, Fair Value Disclosure [Abstract]
Available-for-sale Securities, Debt Securities	$ 0	$ 0