Commitments and Contingencies - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Apr. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Loans Directors	Dec. 31, 2016 USD ($)
Commitments And Contingencies [Line Items]
Commitments expiration period		4 years
Number of loans receivable | Loans		74
Number of independent directors entitled to annual compensation | Directors		5
Loans Receivable [Member]
Commitments And Contingencies [Line Items]
Unfunded loan commitments		$ 1,573,107,000	$ 882,472,000
Five Independent Board of Directors [Member]
Commitments And Contingencies [Line Items]
Annual cash compensation	$ 125,000	175,000
Annual cash compensation paid in cash		75,000
Annual cash compensation paid in the form of deferred stock units		100,000
Chairperson of Audit Committee [Member]
Commitments And Contingencies [Line Items]
Annual cash compensation	$ 12,000	20,000
Audit Committee Members [Member]
Commitments And Contingencies [Line Items]
Annual cash compensation		10,000
Compensation and Corporate Governance Committees [Member]
Commitments And Contingencies [Line Items]
Annual cash compensation		$ 10,000