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Transactions with Related Parties (Detail) € in Millions, £ in Millions, kr in Billions, $ in Billions	3 Months Ended			6 Months Ended
	Jun. 30, 2022 USD ($) shares	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Jun. 30, 2022 USD ($) shares	Jun. 30, 2021 USD ($)	Jun. 30, 2022 AUD ($) shares	Jun. 30, 2022 GBP (£) shares	Dec. 31, 2021 USD ($) shares	Jun. 30, 2021 EUR (€)	Mar. 31, 2021 SEK (kr)
Related Party Transaction [Line Items]
Non-cash expenses				$ 17,068,000	$ 16,105,000
Net book value	$ 24,698,522,000			24,698,522,000				$ 21,878,338,000
B-3 Term Loan
Related Party Transaction [Line Items]
Face Value	$ 417,280,000			$ 417,280,000
Senior Loan Origination Under Marketed Process \| Senior Term Facility
Related Party Transaction [Line Items]
Face amount of loans						$ 5.4	£ 500.0
Senior Loan Origination Under Marketed Process \| Senior Term Facility
Related Party Transaction [Line Items]
Net book value						$ 1.3	£ 250.0
Percentage of financing receivable	0.245			0.245		0.245	0.245
Senior Loan Origination Under Marketed Process \| Senior Term Facility \| Blackstone-Advised Investment Vehicle
Related Party Transaction [Line Items]
Net book value						$ 1.3	£ 250.0
Percentage of financing receivable	0.245			0.245		0.245	0.245
Restricted Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) \| shares	1,621,260			1,621,260		1,621,260	1,621,260	1,706,121
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	$ 27,100,000			$ 27,100,000				$ 28,400,000
Manager
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	27,100,000			27,100,000				$ 28,400,000
Management fees paid to Manager	23,500,000	$ 19,200,000		51,900,000	38,400,000
Expenses reimbursed to Manager	$ 367,000	145,000		$ 560,000	185,000
Manager \| Restricted Class A Common Stock \| Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) \| shares	807,636			807,636		807,636	807,636
Non-cash expenses	$ 4,200,000	4,100,000		$ 8,400,000	8,100,000
Manager \| Restricted Class A Common Stock \| Class A Common Stock \| Manager
Related Party Transaction [Line Items]
Non-cash expenses				$ 25,100,000
Restricted shares, vesting period (in years)				3 years
Affiliates of Manager \| Third-Party Service Provider
Related Party Transaction [Line Items]
Administrative services expenses incurred	95,000	92,000		$ 192,000	192,000
Blackstone-Advised Investment Vehicles, or the Funds \| B-3 Term Loan
Related Party Transaction [Line Items]
Gross financing amount		100,000,000.0			100,000,000.0
Blackstone-Advised Investment Vehicles, or the Funds \| B-4 Term Loan
Related Party Transaction [Line Items]
Debt securities	$ 15,000,000			$ 15,000,000
Percentage of total secured term loans	3.00%			3.00%		3.00%	3.00%
Face Value	$ 500,000,000			$ 500,000,000
Total incentive compensation payments	$ 500,000
Blackstone-Advised Investment Vehicles, or the Funds \| B-3 Term Loan
Related Party Transaction [Line Items]
Debt securities		$ 15,000,000.0			$ 15,000,000.0
Percentage of total secured term loans		15.00%			15.00%				15.00%
Total incentive compensation payments					$ 100,000
Blackstone-Advised Investment Vehicles, or the Funds \| B-1 Term Loan
Related Party Transaction [Line Items]
Debt securities			$ 5,500,000
Percentage of total secured term loans			3.00%							3.00%
Total incentive compensation payments			$ 200,000
Increase in loan			$ 200,000,000
Blackstone-Advised Investment Vehicles, or the Funds \| Senior Term Facility
Related Party Transaction [Line Items]
Face amount of loans \| €									€ 491.0
Senior term facility \| €									€ 50.0
Blackstone-Advised Investment Vehicles, or the Funds \| Senior Term Facility
Related Party Transaction [Line Items]
Net book value \| kr										kr 5.0
Total amount of loans acquired \| kr										kr 10.2
Participation in facility			49.00%							49.00%