Commitments and Contingencies - Additional Information (Detail)		3 Months Ended	12 Months Ended
	Mar. 31, 2017 USD ($) Loans Directors	Mar. 31, 2017 USD ($) Loans Installments	Dec. 31, 2017 USD ($) Directors	Dec. 31, 2016 USD ($)
Commitments And Contingencies [Line Items]
Commitments expiration period		4 years
Number of loans receivable | Loans	58	58
Percentage of compensation paid in cash	40.00%
Percentage of compensation in the form of deferred stock units	60.00%
Number of board of directors | Directors	8
Number of independent directors entitled to annual compensation | Directors	5
Number of equal quarterly installments | Installments		4
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	$ 1,149,842,000	$ 1,149,842,000		$ 882,472,000
Five Independent Board of Directors [Member]
Commitments And Contingencies [Line Items]
Annual cash compensation	125,000
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]
Commitments And Contingencies [Line Items]
Annual cash compensation	$ 12,000
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