Fair Value Measurement and Fair Value of Financial Instruments - Significant Unobservable Inputs Level 3 Assets and Liabilities (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Unobservable Input(s) Level 3
Securities Available for Sale	$ 16,670		$ 16,670		$ 17,789
Direct bank purchase bonds	16,670		16,670
Direct bank purchase bonds
Unobservable Input(s) Level 3
Securities Available for Sale	937		937		$ 936
Direct bank purchase bonds	937		937
Direct bank purchase bonds | Level 3
Unobservable Input(s) Level 3
Securities Available for Sale	$ 937		$ 937
Discount rate | Direct bank purchase bonds | Weighted Average | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.098		0.098
Discount rate | Direct bank purchase bonds | Minimum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0		0
Discount rate | Direct bank purchase bonds | Maximum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0		0
Default Rate | Direct bank purchase bonds | Weighted Average | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.005		0.005
Default Rate | Direct bank purchase bonds | Minimum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0		0
Default Rate | Direct bank purchase bonds | Maximum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0		0
Loss Severity | Direct bank purchase bonds | Weighted Average | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.217		0.217
Loss Severity | Direct bank purchase bonds | Minimum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0		0
Loss Severity | Direct bank purchase bonds | Maximum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0		0
Other Liabilities
Unobservable Input(s) Level 3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	$ 0	$ 0	$ 0	$ 0
Trading derivatives
Unobservable Input(s) Level 3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Trading derivatives
Unobservable Input(s) Level 3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Securities Available for Sale
Unobservable Input(s) Level 3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0	0	0	0
Other Assets
Unobservable Input(s) Level 3
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	$ 0	$ (269)	$ 0	$ (269)