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
	Mar. 31, 2018	Mar. 31, 2017
Estimated Repurchase Liability For Loans Sold [Roll Forward]
Estimate of possible losses in excess of recorded liability	$ 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	$ 12,000,000
Realized losses	0	(1,000,000)
Balance at end of period	10,000,000	$ 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
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