Fair Value Measurement and Fair Value of Financial Instruments - Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Significant unobservable inputs, Level 3 assets
Available-for-sale Securities	$ 16,314	$ 17,563
Direct bank purchase bonds	16,314
Direct bank purchase bonds
Significant unobservable inputs, Level 3 assets
Available-for-sale Securities	1,190	1,503
Direct bank purchase bonds	$ 1,190
Direct bank purchase bonds | Minimum | Return on equity
Significant unobservable inputs, Level 3 assets
Market-required return on capital	8.00%
Probability of default	0.00%
Loss severity	10.00%
Direct bank purchase bonds | Maximum | Return on equity
Significant unobservable inputs, Level 3 assets
Market-required return on capital	10.00%
Probability of default	25.00%
Loss severity	60.00%
Direct bank purchase bonds | Weighted Average | Return on equity
Significant unobservable inputs, Level 3 assets
Market-required return on capital	9.50%
Probability of default	0.30%
Loss severity	22.40%
Level 3 | Direct bank purchase bonds
Significant unobservable inputs, Level 3 assets
Available-for-sale Securities	$ 1,190
Fair Value, Measurements, Recurring
Significant unobservable inputs, Level 3 assets
Direct bank purchase bonds	16,314	17,563
Fair Value, Measurements, Recurring | Direct bank purchase bonds
Significant unobservable inputs, Level 3 assets
Direct bank purchase bonds	1,190	1,503
Fair Value, Measurements, Recurring | Level 3
Significant unobservable inputs, Level 3 assets
Direct bank purchase bonds	1,331	1,599
Fair Value, Measurements, Recurring | Level 3 | Direct bank purchase bonds
Significant unobservable inputs, Level 3 assets
Direct bank purchase bonds	$ 1,190	$ 1,503