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Transactions with Related Parties (Detail) $ in Thousands, € in Millions, £ in Millions, kr in Billions, $ in Billions	3 Months Ended					9 Months Ended
	Sep. 30, 2022 USD ($) shares	Jun. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2022 USD ($) shares	Sep. 30, 2021 USD ($)	Jun. 30, 2022 AUD ($)	Jun. 30, 2022 GBP (£)	Dec. 31, 2021 USD ($) shares	Sep. 30, 2021 GBP (£)	Jun. 30, 2021 EUR (€)	Mar. 31, 2021 SEK (kr)
Related Party Transaction [Line Items]
Non-cash expenses						$ 25,286	$ 24,184
Net book value	$ 24,278,702					$ 24,278,702				$ 21,878,338
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
Total amount of loans acquired			$ 974,500				974,500
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
Restricted Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) \| shares	1,339,445					1,339,445				1,706,121
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	$ 25,900					$ 25,900				$ 28,400
Management fees paid to Manager	27,100		21,500			78,900	59,900
Expenses reimbursed to Manager	$ 163		141			$ 723	325
Manager \| Restricted Class A Common Stock \| Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) \| shares	677,582					677,582
Non-cash expenses	$ 4,100		4,100			$ 12,500	12,200
Manager \| Restricted Class A Common Stock \| Class A Common Stock \| Manager
Related Party Transaction [Line Items]
Non-cash expenses						$ 20,900
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	25,900					$ 25,900				$ 28,400
Affiliates of Manager \| Third-Party Service Provider
Related Party Transaction [Line Items]
Administrative services expenses incurred	88		100			$ 280	291
BTIG, LLC \| Class A Common Stock \| Class A Common Stock
Related Party Transaction [Line Items]
Total incentive compensation payments	$ 191
Blackstone-Advised Investment Vehicles, or the Funds \| B-3 Term Loan
Related Party Transaction [Line Items]
Gross financing amount				$ 100,000
Blackstone-Advised Investment Vehicles, or the Funds \| B-4 Term Loan
Related Party Transaction [Line Items]
Total incentive compensation payments		$ 500
Debt securities		$ 15,000
Percentage of total secured term loans		3.00%						3.00%	3.00%
Face Value		$ 500,000
Blackstone-Advised Investment Vehicles, or the Funds \| B-3 Term Loan
Related Party Transaction [Line Items]
Total incentive compensation payments				100
Debt securities				$ 15,000
Percentage of total secured term loans				15.00%								15.00%
Blackstone-Advised Investment Vehicles, or the Funds \| B-1 Term Loan
Related Party Transaction [Line Items]
Total incentive compensation payments					$ 200
Debt securities					$ 5,500
Percentage of total secured term loans					3.00%								3.00%
Increase in loan					$ 200,000
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			$ 246,600				$ 246,600				£ 186.0		kr 5.0
Participation in facility			49.00%		49.00%		49.00%				49.00%		49.00%
Total amount of loans acquired			$ 503,300				$ 503,300				£ 379.6		kr 10.2
Blackstone-Advised Investment Vehicles, or the Funds \| Senior Loan Origination Under Marketed Process \| Senior Term Facility
Related Party Transaction [Line Items]
Net book value			$ 243,600				$ 243,600
Participation in facility			25.00%				25.00%				25.00%
Blackstone-Advised Investment Vehicles, or the Funds \| Additional Senior Loan Origination Under Marketed Process \| Senior Term Facility
Related Party Transaction [Line Items]
Net book value			$ 243,600				$ 243,600
Participation in facility			25.00%				25.00%				25.00%