Commitments (Unresolved Claims by Claimant) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
GSE
Loss Contingencies Dollars
Balance, beginning of period	$ 1	$ 2
New demands	17	12
Loan paydowns/payoffs		0
Resolved demands	(13)	(13)
Balance, end of period	$ 5	$ 1
Loss Contingencies Units
Balance, beginning of period	6	13
New demands	104	86
Loan paydowns/payoffs		(2)
Resolved demands	(85)	(90)
Balance, end of period	25	7
Private Label
Loss Contingencies Dollars
Balance, beginning of period	$ 0	$ 0
New demands	0	0
Loan paydowns/payoffs		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
Resolved demands	0	0
Balance, end of period	1	0