Commitments (Unresolved Claims by Claimant) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
GSE
Loss Contingencies Dollars
Balance, beginning of period	$ 2	$ 4
New demands	15	22
Loan paydowns/payoffs	0	(1)
Resolved claims	(16)	(23)
Balance, end of period	$ 1	$ 2
Loss Contingencies Units
Balance, beginning of period	13	16
New demands	109	309
Loan paydowns/payoffs	(2)	(8)
Resolved claims	(114)	(304)
Balance, end of period	6	13
Private Label
Loss Contingencies Dollars
Balance, beginning of period	$ 0	$ 0
New demands	0	0
Loan paydowns/payoffs	0	0
Resolved claims	0	0
Balance, end of period	$ 0	$ 0
Loss Contingencies Units
Balance, beginning of period	0	2
New demands	1	4
Loan paydowns/payoffs	0	0
Resolved claims	0	(6)
Balance, end of period	1	0