Commitments and Contingencies - Additional Information (Detail)	9 Months Ended					12 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2024 USD ($) loan	Sep. 30, 2024 USD ($) security_loan	Sep. 30, 2024 USD ($)	Sep. 30, 2024 USD ($) director	Dec. 31, 2023 USD ($) loan	Dec. 31, 2023 USD ($) security_loan	Sep. 30, 2023 USD ($)
Financing Receivable, Allowance for Credit Loss [Line Items]
Current expected credit loss reserves for unfunded loan commitments	$ 8,307,000	$ 8,307,000	$ 8,307,000	$ 8,307,000	$ 8,307,000	$ 15,371,000	$ 15,371,000	$ 13,600,000
Number of loans		149	149			178	178
Number of members of board of directors | director					9
Number of non-employee directors eligible for annual compensation | director					7
Number of directors not eligible for compensation | director					2
Board of Directors
Financing Receivable, Allowance for Credit Loss [Line Items]
Annual cash compensation				210,000
Annual cash compensation paid in cash				95,000
Annual cash compensation paid in the form of deferred stock units				115,000
Audit Committee
Financing Receivable, Allowance for Credit Loss [Line Items]
Annual cash compensation				20,000
Compensation Committee
Financing Receivable, Allowance for Credit Loss [Line Items]
Annual cash compensation				15,000
Corporate Governance Committee
Financing Receivable, Allowance for Credit Loss [Line Items]
Annual cash compensation				10,000
Audit Committee Members
Financing Receivable, Allowance for Credit Loss [Line Items]
Annual cash compensation				10,000
Investment risk management committee
Financing Receivable, Allowance for Credit Loss [Line Items]
Annual cash compensation				7,500
Unfunded Loan Commitment
Financing Receivable, Allowance for Credit Loss [Line Items]
Current expected credit loss reserves for unfunded loan commitments	1,562,589,000	$ 1,562,589,000	$ 1,562,589,000	1,562,589,000	$ 1,562,589,000	$ 2,430,664,000	$ 2,430,664,000
Number of loans | loan		76
Aggregate unfunded loan commitments	717,900,000	$ 717,900,000	717,900,000	717,900,000	717,900,000
Net unfunded commitments	$ 844,700,000	$ 844,700,000	$ 844,700,000	$ 844,700,000	$ 844,700,000
Weighted-average future funding period (in years)	2 years 3 months 18 days