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 ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Decrease in liability recorded through earnings	$ 522,000,000
Estimate of possible losses in excess of recorded liability	350,000,000
Obligation to Repurchase Receivables Sold [Member]
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Balance at beginning of period	10,000,000	$ 12,000,000	$ 17,000,000
Decrease in liability recorded through earnings	(1,000,000)	(1,000,000)	(3,000,000)
Realized losses	(1,000,000)	(1,000,000)	(2,000,000)
Balance at end of period	8,000,000	$ 10,000,000	$ 12,000,000
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
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