Transactions with Related Parties (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022	Dec. 31, 2022
Related Party Transaction [Line Items]
Non-cash expenses	$ 7,655	$ 8,650
Restricted Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares)	2,198,627		1,883,784
Restricted shares, vesting period (in years)	3 years
Manager
Related Party Transaction [Line Items]
Renewal term (in years)	1 year
Accrued management and incentive fees payable	$ 31,100		$ 33,800
Management fees paid to Manager	33,800	28,400
Expenses reimbursed to Manager	$ 382	193
Manager | Restricted Class A Common Stock | Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares)	1,092,795
Non-cash expenses	$ 3,900	4,200
Manager | Restricted Class A Common Stock | Class A Common Stock | Manager
Related Party Transaction [Line Items]
Non-cash expenses	$ 29,700
Restricted shares, vesting period (in years)	3 years
BXMT Advisors Limited Liability Company and Affiliates
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	$ 31,100		$ 33,800
Affiliates of Manager | Third-Party Service Provider
Related Party Transaction [Line Items]
Administrative services expenses incurred	$ 238	$ 97