Fair Value Measurement and Fair Value of Financial Instruments - Level 3 Assets and Liabilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Significant unobservable inputs, Level 3 assets
Direct bank purchase bonds	$ 17,553
Direct bank purchase bonds
Significant unobservable inputs, Level 3 assets
Direct bank purchase bonds	$ 1,503
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	9.70%
Probability of default	0.30%
Loss severity	27.60%
Fair Value, Measurements, Recurring
Significant unobservable inputs, Level 3 assets
Direct bank purchase bonds	$ 17,563	$ 14,141
Fair Value, Measurements, Recurring | Direct bank purchase bonds
Significant unobservable inputs, Level 3 assets
Direct bank purchase bonds	1,503	1,613
Fair Value, Measurements, Recurring | Level 3
Significant unobservable inputs, Level 3 assets
Direct bank purchase bonds	1,599	1,638
Fair Value, Measurements, Recurring | Level 3 | Direct bank purchase bonds
Significant unobservable inputs, Level 3 assets
Direct bank purchase bonds	$ 1,503	$ 1,613