Commitments (Unresolved Claims by Claimant) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
GSE
Loss Contingencies Dollars
Balance, beginning of period	$ 2	$ 4
New demands	3	6
Loan paydowns/payoffs	0	(1)
Resolved demands	(4)	(7)
Balance, end of period	$ 1	$ 2
Loss Contingencies Units
Balance, beginning of period	13	16
New demands	23	71
Loan paydowns/payoffs	(1)	(4)
Resolved demands	(24)	(68)
Balance, end of period	11	15
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	2
Loan paydowns/payoffs	0	0
Resolved demands	0	(3)
Balance, end of period	0	1