Fair Value Measurements (Assets Measured on a Nonrecurring Basis, Level 3 Quantitative Information) (Details) - Nonrecurring - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Repossessed assets	$ 9	$ 14
Residential first mortgage loans
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans held-for-investment	21	25
Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Repossessed assets	$ 9	$ 14
Level 3 | Fair value of underlying collateral | Lower range
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Loss severity (as a percent)	12.00%	22.00%
Level 3 | Fair value of underlying collateral | Upper range
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Loss severity (as a percent)	100.00%	100.00%
Level 3 | Fair value of underlying collateral | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Loss severity (as a percent)	73.80%	69.50%
Level 3 | Residential first mortgage loans
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans held-for-investment	$ 21	$ 25
Level 3 | Residential first mortgage loans | Fair value of underlying collateral
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans held-for-investment	$ 21	$ 25
Level 3 | Residential first mortgage loans | Fair value of underlying collateral | Lower range
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Loss severity (as a percent)	27.00%	22.00%
Level 3 | Residential first mortgage loans | Fair value of underlying collateral | Upper range
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Loss severity (as a percent)	29.00%	40.00%
Level 3 | Residential first mortgage loans | Fair value of underlying collateral | Weighted Average
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Loss severity (as a percent)	28.00%	29.50%
Level 3 | Repossessed assets | Fair value of underlying collateral
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Repossessed assets	$ 9	$ 14