Commitments (Unresolved Claims by Claimant) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
GSE
Loss Contingencies Dollars
Balance, beginning of period	$ 1	$ 1
New demands	45	19
Loan paydowns/payoffs	0
Resolved claims	(40)	(19)
Balance, end of period	$ 6	$ 1
Loss Contingencies Units
Balance, beginning of period	9	6
New demands	258	121
Loan paydowns/payoffs	(3)
Resolved claims	(237)	(118)
Balance, end of period	27	9
Private Label
Loss Contingencies Dollars
Balance, beginning of period	$ 0	$ 0
New demands	1	0
Loan paydowns/payoffs	0
Resolved claims	(1)	0
Balance, end of period	$ 0	$ 0
Loss Contingencies Units
Balance, beginning of period	1	1
New demands	8	0
Loan paydowns/payoffs	0
Resolved claims	(8)	0
Balance, end of period	1	1