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 ($)	Sep. 30, 2020	Dec. 31, 2019
Loss Contingencies [Line Items]
Estimate of possible losses in excess of recorded liability	$ 150,000,000
Obligation to Repurchase Receivables Sold [Member]
Loss Contingencies [Line Items]
Estimated repurchase liability	3,000,000	$ 4,000,000
Minimum [Member] | Obligation to Repurchase Receivables Sold [Member]
Loss Contingencies [Line Items]
Estimate of possible losses in excess of recorded liability	0
Maximum [Member] | Obligation to Repurchase Receivables Sold [Member]
Loss Contingencies [Line Items]
Estimate of possible losses in excess of recorded liability	$ 25,000,000