Fair Value Measurement and Fair Value of Financial Instruments - Significant Unobservable Inputs Level 3 Assets and Liabilities (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Unobservable Input(s) Level 3
Securities Available for Sale	$ 21,125		$ 21,125		$ 18,259
Direct bank purchase bonds	21,125		21,125
Direct bank purchase bonds
Unobservable Input(s) Level 3
Securities Available for Sale	708		708		930
Direct bank purchase bonds	708		708
Direct bank purchase bonds | Level 3
Unobservable Input(s) Level 3
Securities Available for Sale	708		708
Other
Unobservable Input(s) Level 3
Securities Available for Sale	527		527		$ 539
Direct bank purchase bonds	527		527
Other | Level 3
Unobservable Input(s) Level 3
Securities Available for Sale	$ 512		$ 512
Discount rate | Direct bank purchase bonds | Weighted Average | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.100		0.100
Discount rate | Direct bank purchase bonds | Minimum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.080		0.080
Discount rate | Direct bank purchase bonds | Maximum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.100		0.100
Discount rate | Other | Weighted Average | Valuation Technique, Discounted Cash Flow
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.035		0.035
Discount rate | Preferred Stock | Minimum | Valuation Technique, Discounted Cash Flow
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	1.1		1.1
Discount rate | Preferred Stock | Maximum | Valuation Technique, Discounted Cash Flow
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	4.5		4.5
Default Rate | Direct bank purchase bonds | Weighted Average | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.003		0.003
Default Rate | Direct bank purchase bonds | Minimum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.001		0.001
Default Rate | Direct bank purchase bonds | Maximum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.041		0.041
Loss Severity | Direct bank purchase bonds | Weighted Average | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.176		0.176
Loss Severity | Direct bank purchase bonds | Minimum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.100		0.100
Loss Severity | Direct bank purchase bonds | Maximum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.450		0.450
Measurement Input, Liquidity Adjustment | Other | Weighted Average | Valuation Technique, Discounted Cash Flow
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.022		0.022
Measurement Input, Liquidity Adjustment | Preferred Stock | Minimum | Valuation Technique, Discounted Cash Flow
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.8		0.8
Measurement Input, Liquidity Adjustment | Preferred Stock | Maximum | Valuation Technique, Discounted Cash Flow
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	2.9		2.9
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	$ 0	$ 42	$ 0