Commitments (Unresolved Claims by Claimant) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
GSE
Loss Contingencies Dollars
Balance, beginning of period	$ 2	$ 4
New demands	12	18
Loan paydowns/payoffs	0	(1)
Resolved demands	(13)	(14)
Balance, end of period	$ 1	$ 7
Loss Contingencies Units
Balance, beginning of period	13	16
New demands	86	253
Loan paydowns/payoffs	(2)	(8)
Resolved demands	(90)	(203)
Balance, end of period	7	58
Private Label
Loss Contingencies Dollars
Balance, beginning of period	$ 0	$ 0
New demands	0	0
Loan paydowns/payoffs	0	0
Resolved demands	0	0
Balance, end of period	$ 0	$ 0
Loss Contingencies Units
Balance, beginning of period	0	2
New demands	0	4
Loan paydowns/payoffs	0	0
Resolved demands	0	(6)
Balance, end of period	0	0