Fair Value Measurements - Carrying Amount and Estimated Fair Value of Financial Instruments (Details) - Nonrecurring - Level 3 $ in Millions	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Residential first mortgage loans
Fair Value Inputs, Assets, Quantitative Information
Loans held-for-investment	$ 14	$ 12
Residential first mortgage loans | Loss severity discount | Lower range
Fair Value Inputs, Assets, Quantitative Information
Loans held for investment, fair value input	0.25	0.25
Residential first mortgage loans | Loss severity discount | Upper range
Fair Value Inputs, Assets, Quantitative Information
Loans held for investment, fair value input	0.30	0.30
Residential first mortgage loans | Loss severity discount | Weighted Average
Fair Value Inputs, Assets, Quantitative Information
Loans held for investment, fair value input	0.254	0.283
Repossessed assets
Fair Value Inputs, Assets, Quantitative Information
Loans held-for-investment	$ 9	$ 7
Repossessed assets | Loss severity discount | Lower range
Fair Value Inputs, Assets, Quantitative Information
Repossessed asset, fair value input	0.00	0.00
Repossessed assets | Loss severity discount | Upper range
Fair Value Inputs, Assets, Quantitative Information
Repossessed asset, fair value input	1.00	1.00
Repossessed assets | Loss severity discount | Weighted Average
Fair Value Inputs, Assets, Quantitative Information
Repossessed asset, fair value input	0.207	0.258