Fair Value Measurement and Fair Value of Financial Instruments - Significant Unobservable Inputs Level 3 Assets and Liabilities (Details)	Mar. 31, 2020 USD ($)
Unobservable Input(s) Level 3
Direct bank purchase bonds	$ 15,140,000,000
Direct bank purchase bonds
Unobservable Input(s) Level 3
Direct bank purchase bonds	940,000,000
Direct bank purchase bonds | Level 3
Unobservable Input(s) Level 3
Direct bank purchase bonds	$ 940,000,000
Measurement Input, Discount Rate [Member] | Direct bank purchase bonds | Weighted Average [Member] | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.099
Measurement Input, Discount Rate [Member] | Direct bank purchase bonds | Minimum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.080
Measurement Input, Discount Rate [Member] | Direct bank purchase bonds | Maximum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.100
Measurement Input, Default Rate [Member] | Direct bank purchase bonds | Weighted Average [Member] | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.004
Measurement Input, Default Rate [Member] | Direct bank purchase bonds | Minimum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.000
Measurement Input, Default Rate [Member] | Direct bank purchase bonds | Maximum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.250
Measurement Input, Loss Severity [Member] | Direct bank purchase bonds | Weighted Average [Member] | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.201
Measurement Input, Loss Severity [Member] | Direct bank purchase bonds | Minimum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.100
Measurement Input, Loss Severity [Member] | Direct bank purchase bonds | Maximum | Return on equity
Unobservable Input(s) Level 3
Direct bank purchase bonds, measurement input	0.450