Transactions with Related Parties (Detail) $ in Thousands, £ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) shares	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($) shares	Mar. 31, 2019 GBP (£)
Related Party Transaction [Line Items]
Non-cash expenses			$ 22,926	$ 25,286
Senior Term Facility
Related Party Transaction [Line Items]
Senior term facility | £						£ 240.1
Face amount of loans | £						£ 490.0
Restricted Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) | shares	1,539,475		1,539,475		1,883,784
Restricted shares, vesting period (in years)			3 years
Related Party | Manager
Related Party Transaction [Line Items]
Renewal term (in years)			1 year
Management fees paid to Manager	$ 32,800	$ 27,100	$ 97,700	78,900
Expenses reimbursed to Manager	325	163	2,300	723
Related Party | BXMT Advisors Limited Liability Company and Affiliates
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	28,900		28,900		$ 33,800
Related Party | Third-Party Service Provider
Related Party Transaction [Line Items]
Administrative services expenses incurred	$ 320	88	$ 800	280
Related Party | Restricted Class A Common Stock | Class A Common Stock | Manager
Related Party Transaction [Line Items]
Shares held (in shares) | shares	762,845		762,845
Non-cash expenses	$ 3,900	$ 4,100	$ 11,700	$ 12,500
Related Party | Restricted Class A Common Stock | Class A Common Stock | Manager | Manager
Related Party Transaction [Line Items]
Non-cash expenses			$ 20,600
Restricted shares, vesting period (in years)			3 years