Commitments and Contingencies - Additional Information (Detail)	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) Loans	Dec. 31, 2017 USD ($) Directors	Dec. 31, 2016 USD ($)
Commitments And Contingencies [Line Items]
Commitments expiration period	4 years
Number of loans receivable | Loans	67
Scenario, Forecast [Member]
Commitments And Contingencies [Line Items]
Number of independent directors entitled to annual compensation | Directors		5
Loans Receivable [Member]
Commitments And Contingencies [Line Items]
Unfunded loan commitments	$ 1,622,216,000		$ 882,472,000
Five Independent Board of Directors [Member] | Scenario, Forecast [Member]
Commitments And Contingencies [Line Items]
Annual cash compensation		$ 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] | Scenario, Forecast [Member]
Commitments And Contingencies [Line Items]
Annual cash compensation		20,000
Audit Committee Members [Member] | Scenario, Forecast [Member]
Commitments And Contingencies [Line Items]
Annual cash compensation		10,000
Compensation and Corporate Governance Committees [Member] | Scenario, Forecast [Member]
Commitments And Contingencies [Line Items]
Annual cash compensation		$ 10,000