Commitments and Contingencies - Additional Information (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Loans Directors	Mar. 31, 2018 USD ($) Loans	Dec. 31, 2017 USD ($)
Commitments And Contingencies [Line Items]
Commitments expiration period		5 years
Number of loans receivable | Loans	74	74
Number of independent directors entitled to annual compensation | Directors	5
Loans Receivable [Member]
Commitments And Contingencies [Line Items]
Unfunded loan commitments	$ 1,473,112,000	$ 1,473,112,000	$ 1,573,107,000
Five Independent Board of Directors [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	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