Commitments (Unresolved Claims by Claimant) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
GSE
Loss Contingencies Dollars
Balance, beginning of period	$ 1	$ 2
New demands	5	3
Loan paydowns/payoffs	0	0
Resolved demands	(4)	(4)
Balance, end of period	$ 2	$ 1
Loss Contingencies Units
Balance, beginning of period	6	13
New demands	30	23
Loan paydowns/payoffs	0	(1)
Resolved demands	(22)	(24)
Balance, end of period	14	11
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	1	0
New demands	0	0
Loan paydowns/payoffs	0	0
Resolved demands	0	0
Balance, end of period	1	0