Transactions With Related Parties - Additional Information (Detail) $ in Thousands, € in Millions, £ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2024 USD ($) shares	Sep. 30, 2024 EUR (€)	Sep. 30, 2024 GBP (£)	Jun. 30, 2024 GBP (£)	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) shares	Sep. 30, 2023 USD ($)	Sep. 30, 2024 EUR (€) shares	Sep. 30, 2024 GBP (£) shares	Dec. 31, 2023 USD ($) shares	Dec. 31, 2021 EUR (€)	Dec. 31, 2019 EUR (€)	Mar. 31, 2019 GBP (£)	Dec. 31, 2018 GBP (£)
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	$ 18,600						$ 18,600				$ 26,300
Non-cash expenses	$ 7,984					$ 7,606	$ 24,058	$ 22,926
Common stock, shares outstanding (in shares) | shares	172,987,597						172,987,597		172,987,597	172,987,597	173,209,933
Repayment of loan participations							$ 235,960	0
Principal collections, sales proceeds, and cost-recovery proceeds from loans receivable							$ 3,135,968	2,323,258
Restricted Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) | shares	1,627,201						1,627,201		1,627,201	1,627,201	2,180,181
Restricted shares, vesting period (in years)							3 years
Related Party
Related Party Transaction [Line Items]
Common stock, shares outstanding (in shares) | shares	12,390,063						12,390,063		12,390,063	12,390,063
Percentage of stock	7.20%						7.20%		7.20%	7.20%
Face amount of loans	$ 94,400						$ 94,400			£ 490.0		€ 350.0	€ 350.0	£ 240.1	£ 148.7
Related party transaction, interest in loan, percentage	17.00%	17.00%	17.00%
Selling loan amount | €		€ 232.0
Remaining loan amount in new lenders | €									€ 347.0
Repayment of loan participations | £			£ 46.4
Principal collections, sales proceeds, and cost-recovery proceeds from loans receivable					$ 98,600
Related Party | Senior Loans
Related Party Transaction [Line Items]
Face amount of loans	$ 560,000						$ 560,000			£ 303.5
Related Party | Senior Participations
Related Party Transaction [Line Items]
Repayment of loan participations | £				£ 186.0
Proceeds of loan participations | £			£ 100.0
Related Party | Junior Loan Participation
Related Party Transaction [Line Items]
Repayment of loan participations | £				£ 46.5
Related Party | Manager
Related Party Transaction [Line Items]
Renewal term (in years)							1 year
Management fees paid to Manager	18,700					32,800	$ 64,000	97,700
Expenses	340					325	1,400	2,300
Related Party | BXMT Advisors Limited Liability Company and Affiliates
Related Party Transaction [Line Items]
Management fee payable	18,600						18,600
Incentive fee payable	$ 0						$ 0
Accrued management and incentive fees payable											$ 26,300
Related Party | Restricted Class A Common Stock | Manager
Related Party Transaction [Line Items]
Shares held (in shares) | shares	829,035						829,035		829,035	829,035
Aggregate grant fair value							$ 19,600
Restricted shares, vesting period (in years)							3 years
Non-cash expenses	$ 4,200					$ 3,900	$ 12,600	$ 11,700
Subsidiary of Common Parent
Related Party Transaction [Line Items]
Common stock, shares outstanding (in shares) | shares	7,582,044						7,582,044		7,582,044	7,582,044
Percentage of stock	4.40%						4.40%		4.40%	4.40%
Director | Deferred Stock Units
Related Party Transaction [Line Items]
Shares held (in shares) | shares	401,802						401,802		401,802	401,802