Commitments (Unresolved Claims by Claimant) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)
GSE
Loss Contingencies Dollars
Balance, beginning of period	$ 1	$ 1
New demands	18	5
Resolved demands	(13)	(4)
Balance, end of period	$ 6	$ 2
Loss Contingencies Units
Balance, beginning of period	9	6
New demands	95	30
Resolved demands	(71)	(22)
Balance, end of period	33	14
Private Label
Loss Contingencies Dollars
Balance, beginning of period	$ 0	$ 0
New demands	0	0
Resolved demands	0	0
Balance, end of period	$ 0	$ 0
Loss Contingencies Units
Balance, beginning of period	1	1
New demands	0	0
Resolved demands	(1)	0
Balance, end of period	0	1