Loans Held for Sale, at Fair Value - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Accounts Notes And Loans Receivable [Line Items]
Maximum number of days loans held for sale are typically sold			45 days
Loans held for sale in nonaccrual status	$ 0		$ 0		$ 0
Management Services, Servicing Fees and Other [Member]
Accounts Notes And Loans Receivable [Line Items]
Interest income on Loans held for sale	14,800,000	$ 10,300,000	29,900,000	$ 19,900,000
Gains (Loss) from Mortgage Banking Activities, Net [Member]
Accounts Notes And Loans Receivable [Line Items]
Gains/(losses) recognized for fair value adjustments on loans held for sale	(3,200,000)	$ (6,800,000)	22,500,000	$ 500,000
Greater Than 90 Days Past Due [Member]
Accounts Notes And Loans Receivable [Line Items]
Loans held for sale past due	$ 0		$ 0		$ 0