Commitments (Unresolved Claims by Claimant) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
GSE
Loss Contingencies Dollars
Balance, beginning of period	$ 1	$ 2
New demands	19	15
Loan paydowns/payoffs	0	0
Resolved claims	(19)	(16)
Balance, end of period	$ 1	$ 1
Loss Contingencies Units
Balance, beginning of period	6	13
New demands	121	109
Loan paydowns/payoffs	0	(2)
Resolved claims	(118)	(114)
Balance, end of period	9	6
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	1	0
New demands	0	1
Loan paydowns/payoffs	0	0
Resolved claims	0	0
Balance, end of period	1	1