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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Decrease in liability recorded through earnings	$ 3,000,000		$ 513,000,000
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	9,000,000	$ 11,000,000	10,000,000	$ 12,000,000
Decrease in liability recorded through earnings	0	0	(1,000,000)	(1,000,000)
Realized losses	(1,000,000)	(1,000,000)	(1,000,000)	(1,000,000)
Balance at end of period	8,000,000	$ 10,000,000	8,000,000	$ 10,000,000
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