Commitments (Unresolved Claims by Claimant) (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)
GSE
Loss Contingencies Dollars
Balance, beginning of period	$ 1	$ 2
New demands	11	8
Loan paydowns/payoffs		0
Resolved demands	(9)	(8)
Balance, end of period	$ 3	$ 2
Loss Contingencies Units
Balance, beginning of period	6	13
New demands	62	58
Loan paydowns/payoffs		(1)
Resolved demands	(54)	(54)
Balance, end of period	14	16
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	0
New demands	0	0
Loan paydowns/payoffs		0
Resolved demands	0	0
Balance, end of period	1	0