Commitments and Contingencies - Repurchase and Foreclosure-Related Reserves (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016
Changes in repurchase and foreclosure-related reserves
Balance, beginning of period	$ 73	$ 89
Realized losses	(34)	(42)
Transfer of reserves	(5)	0
Increase in reserves due to:
Changes in assumptions	3	19
New loan sales	3	7
Balance, end of period	73	89	$ 40	$ 73
Components of Repurchase and Foreclosure-related Reserves [Abstract]
Loan repurchase and indemnification liability			29	49
Adjustment to value for real estate owned			9	14
Allowance for probable foreclosure losses			2	10
Total	$ 73	$ 89	$ 40	$ 73