Commitments - Unresolved Claims by Claimant (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)
GSE
Loss Contingencies Dollars
Balance, beginning of period	$ 1	$ 1
New demands	27	11
Loan paydowns/payoffs	0
Resolved demands	23	9
Balance, end of period	$ 5	$ 3
Loss Contingencies Units
Balance, beginning of period	9	6
New demands	160	62
Loan paydowns/payoffs	3
Resolved demands	(139)	(54)
Balance, end of period	27	14
Private Label
Loss Contingencies Dollars
Balance, beginning of period	$ 0	$ 0
New demands	0	0
Loan paydowns/payoffs	0
Resolved demands	0	0
Balance, end of period	$ 0	$ 0
Loss Contingencies Units
Balance, beginning of period	1	1
New demands	1	0
Loan paydowns/payoffs	0
Resolved demands	(2)	0
Balance, end of period	0	1