Variable Interest Entities - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Jul. 31, 2018	Dec. 31, 2017	Jun. 30, 2017
Variable Interest Entity [Line Items]
Loan contributed to securitization	$ 512,002
Subordinate risk retention interest notional amount	99,000
Single asset securitization principal amount		$ 1,000,000		$ 474,600
Loans Receivable [Member]
Variable Interest Entity [Line Items]
Loan contributed to securitization	517,500
2018 Single Asset Securitization [Member]
Variable Interest Entity [Line Items]
Single asset securitization principal amount	1,000,000
Maximum exposure to loss	96,200
Collateral Assets [Member]
Variable Interest Entity [Line Items]
Single asset securitization principal amount	1,682,297		$ 1,656,406
Collateral Assets [Member] | Collateralized Loan Obligations [Member]
Variable Interest Entity [Line Items]
Single asset securitization principal amount	$ 1,000,000		$ 1,000,000