Guarantee Arrangements, Pledged Assets and Repurchase Agreements - Summary of Change in Estimated Repurchase Liability for Loans Sold to GSEs and Other Third Parties (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Decrease in liability recorded through earnings	$ 16,000,000		$ 510,000,000
Estimate of possible losses in excess of recorded liability	325,000,000		325,000,000
Obligation to Repurchase Receivables Sold [Member]
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Balance at beginning of period	10,000,000	$ 11,000,000	10,000,000	$ 12,000,000
Decrease in liability recorded through earnings	(1,000,000)	(1,000,000)	(1,000,000)	(1,000,000)
Realized losses	0	1,000,000	0	0
Balance at end of period	9,000,000	$ 11,000,000	9,000,000	$ 11,000,000
Minimum [Member] | Obligation to Repurchase Receivables Sold [Member]
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Estimate of possible losses in excess of recorded liability	0		0
Maximum [Member] | Obligation to Repurchase Receivables Sold [Member]
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Estimate of possible losses in excess of recorded liability	$ 25,000,000		$ 25,000,000