Variable Interest Entities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2019	Sep. 30, 2019
Variable Interest Entity [Line Items]
Loan contributed to securitization		$ 512,002
Subordinate risk retention interest notional amount	$ 99,000
Loans Receivable [Member]
Variable Interest Entity [Line Items]
Loan contributed to securitization	517,500	(517,500)
2018 Single Asset Securitization [Member]
Variable Interest Entity [Line Items]
Single asset securitization principal amount	$ 1,000,000	$ 1,000,000	$ 930,000	$ 1,000,000
Maximum exposure to loss			$ 86,600