Derivative Financial Instruments - Offsetting Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Offsetting Assets and Liabilities [Line Items]
Derivatives, Asset Fair Value	$ 396	$ 737
Derivatives, Liability Fair Value	148	295
Counterparty netting, Assets	(100)	(250)
Counterparty netting, Liabilities	(100)	(250)
Cash collateral netting, Assets	(251)	(399)
Cash collateral netting, Liabilities	0	(6)
Securities collateral netting, Assets	(37)	(20)
Securities collateral netting, Liabilities	(40)	(14)
Net receivables/payables, Assets	8	68
Net receivables/payables, Liabilities	8	25
Equity contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	1,382	782
Derivatives, Asset Fair Value	197	94
Derivatives, Liability Fair Value	19	0
Foreign exchange contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	710	692
Derivatives, Asset Fair Value	0	58
Derivatives, Liability Fair Value	62	34
Interest rate contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	24,490	32,898
Derivatives, Asset Fair Value	173	555
Derivatives, Liability Fair Value	57	245
Credit contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	1,983	3,051
Derivatives, Asset Fair Value	26	30
Derivatives, Liability Fair Value	10	16
Held for sale
Offsetting Assets and Liabilities [Line Items]
Derivatives, Asset Fair Value	1,495	954
Derivatives, Liability Fair Value	780	173
Counterparty netting, Assets	(776)	(161)
Counterparty netting, Liabilities	(776)	(161)
Cash collateral netting, Assets	(676)	(685)
Cash collateral netting, Liabilities	(4)	(15)
Securities collateral netting, Assets	(31)	(52)
Securities collateral netting, Liabilities	0	0
Net receivables/payables, Assets	12	56
Net receivables/payables, Liabilities	0	(3)
Held for sale | Equity contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	28,131	21,545
Derivatives, Asset Fair Value	1,023	378
Derivatives, Liability Fair Value	662	49
Held for sale | Foreign exchange contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	244	1,362
Derivatives, Asset Fair Value	0	43
Derivatives, Liability Fair Value	24	16
Held for sale | Interest rate contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	27,025	35,444
Derivatives, Asset Fair Value	471	530
Derivatives, Liability Fair Value	88	108
Held for sale | Credit contracts
Offsetting Assets and Liabilities [Line Items]
Derivative, Notional Amount	431	204
Derivatives, Asset Fair Value	1	3
Derivatives, Liability Fair Value	$ 6	$ 0