Fair Value Measurement and Fair Value of Financial Instruments - Level 3 Assets and Liabilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Reconciliation of assets measured using significant unobservable inputs
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 8
Available-for-sale Securities	17,789	$ 16,314
Direct bank purchase bonds	17,789
Direct bank purchase bonds
Reconciliation of assets measured using significant unobservable inputs
Available-for-sale Securities	936	1,190
Direct bank purchase bonds	936
Level 3 | Direct bank purchase bonds
Reconciliation of assets measured using significant unobservable inputs
Available-for-sale Securities	936
Fair Value, Measurements, Recurring | Level 3
Reconciliation of assets measured using significant unobservable inputs
Direct bank purchase bonds	1,131	1,331
Fair Value, Measurements, Recurring | Level 3 | Direct bank purchase bonds
Reconciliation of assets measured using significant unobservable inputs
Direct bank purchase bonds	$ 936	1,190
Measurement Input, Discount Rate [Member] | Direct bank purchase bonds | Minimum | Return on equity
Reconciliation of assets measured using significant unobservable inputs
Market-required return on capital	0
Measurement Input, Discount Rate [Member] | Direct bank purchase bonds | Maximum | Return on equity
Reconciliation of assets measured using significant unobservable inputs
Market-required return on capital	0
Measurement Input, Discount Rate [Member] | Direct bank purchase bonds | Weighted Average | Return on equity
Reconciliation of assets measured using significant unobservable inputs
Market-required return on capital	0.094
Measurement Input, Default Rate [Member] | Direct bank purchase bonds | Minimum | Return on equity
Reconciliation of assets measured using significant unobservable inputs
Market-required return on capital	0
Measurement Input, Default Rate [Member] | Direct bank purchase bonds | Maximum | Return on equity
Reconciliation of assets measured using significant unobservable inputs
Market-required return on capital	0
Measurement Input, Default Rate [Member] | Direct bank purchase bonds | Weighted Average | Return on equity
Reconciliation of assets measured using significant unobservable inputs
Market-required return on capital	0.003
Measurement Input, Loss Severity [Member] | Direct bank purchase bonds | Minimum | Return on equity
Reconciliation of assets measured using significant unobservable inputs
Market-required return on capital	0
Measurement Input, Loss Severity [Member] | Direct bank purchase bonds | Maximum | Return on equity
Reconciliation of assets measured using significant unobservable inputs
Market-required return on capital	0
Measurement Input, Loss Severity [Member] | Direct bank purchase bonds | Weighted Average | Return on equity
Reconciliation of assets measured using significant unobservable inputs
Market-required return on capital	0.200
Securities Available for Sale
Reconciliation of assets measured using significant unobservable inputs
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	$ 1,131	1,331	$ 1,600
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	31	(9)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	57	126
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(288)	(386)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	0	0
Other Assets
Reconciliation of assets measured using significant unobservable inputs
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	316	277	65
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(88)	(8)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	396	220
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(269)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	0	0
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	(103)	(8)
Trading Derivatives [Member]
Reconciliation of assets measured using significant unobservable inputs
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		13	139
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		(10)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements		(116)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)		(10)
Trading Assets, Excluding Debt and Equity Securities [Member]
Reconciliation of assets measured using significant unobservable inputs
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		13
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Settlements	(9)
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	4
Other Liabilities
Reconciliation of assets measured using significant unobservable inputs
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	(124)	(118)	(119)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	(6)	1
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	0	0
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)	(6)	1
Trading Derivatives [Member]
Reconciliation of assets measured using significant unobservable inputs
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value	(8)	(13)	$ (138)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings		9
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases		0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		116
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)		9
Trading Liabilities [Member]
Reconciliation of assets measured using significant unobservable inputs
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value		$ (13)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Earnings	(4)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Gain (Loss) Included in Other Comprehensive Income (Loss)	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Purchases	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	9
Fair Value, Liability, Recurring Basis, Still Held, Unrealized Gain (Loss)	$ (4)