Commitments and Contingencies - Repurchase and Foreclosure-Related Reserves (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Changes in repurchase and foreclosure-related reserves
Balance, beginning of period	$ 40	$ 73
Realized losses	(4)	(13)
Increase (decrease) in reserves due to:
Changes in assumptions	(1)	2
New loan sales	0	1
Balance, end of period	40	73	$ 35	$ 40	$ 63
Components of Repurchase and Foreclosure-related Reserves [Abstract]
Loan repurchase and indemnification liability			27	29
Adjustment to value for real estate owned			6	9
Allowance for probable foreclosure losses			2	2
Total	$ 40	$ 73	35	$ 40	$ 63
Loan Repurchases and Indemnifications
Loans with full risk of loss or with respect to which the company has identified a breach of representation and warranty provisions			$ 39
Percentage of loans with full risk of loss or with respect to which the company has identified a breach of representation and warranty provisions and are at least 90 days delinquent			20.00%