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Transactions with Related Parties (Detail) $ in Thousands, € in Millions, £ in Millions, $ in Millions, kr in Billions	3 Months Ended					9 Months Ended	12 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2021 EUR (€)	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2022 USD ($) shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2021 USD ($) loan shares	Dec. 31, 2020 USD ($) loan shares	Jun. 30, 2022 AUD ($)	Jun. 30, 2022 GBP (£)	Dec. 31, 2021 AUD ($) shares	Sep. 30, 2021 USD ($)	Sep. 30, 2021 GBP (£)	Jun. 30, 2021 EUR (€)	Mar. 31, 2021 SEK (kr)	Dec. 31, 2019 EUR (€)	Sep. 30, 2019 EUR (€)
Related Party Transaction [Line Items]
Non-cash expenses							$ 33,414	$ 31,647	$ 34,532
Net book value						$ 24,691,743	24,691,743	$ 21,878,338
Face amount of loans						$ 25,160,343	$ 25,160,343
Senior Term Facility
Related Party Transaction [Line Items]
Participation in facility																	16.00%	16.00%
Additional interest in loans \| €		€ 100.0
Percentage ownership on loan	22.00%														22.00%
Senior Loan Origination Under Marketed Process \| Senior Term Facility
Related Party Transaction [Line Items]
Face amount of loans										$ 5,400.0	£ 500.0
Senior Loan Origination Under Marketed Process \| Senior Term Facility
Related Party Transaction [Line Items]
Net book value										$ 1,300.0	£ 250.0
Percentage of financing receivable										0.245	0.245
Total amount of loans acquired													$ 974,500
Senior Loan Origination Under Marketed Process \| Senior Term Facility \| Blackstone-Advised Investment Vehicle
Related Party Transaction [Line Items]
Net book value										$ 1,300.0	£ 250.0
Percentage of financing receivable										0.245	0.245
Restricted Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) \| shares						1,883,784	1,883,784	1,706,121	1,627,890			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						$ 33,800	$ 33,800	$ 28,400
Management fees paid to Manager							104,800	79,300	$ 78,900
Shares issued for management and incentive fees (in shares) \| shares									840,696
Expenses reimbursed to Manager							$ 896	601	$ 1,000
Manager \| Class A Common Stock
Related Party Transaction [Line Items]
Management fees paid to Manager					$ 19,300
Shares issued for management and incentive fees (in shares) \| shares									840,696
Manager \| Restricted Class A Common Stock \| Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) \| shares						1,178,855	1,178,855
Non-cash expenses							$ 16,600	15,300	$ 17,000
Manager \| Restricted Class A Common Stock \| Class A Common Stock \| Manager
Related Party Transaction [Line Items]
Non-cash expenses							$ 32,300
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						$ 33,800	$ 33,800	$ 28,400
Affiliates of Manager
Related Party Transaction [Line Items]
Number of loans originated \| loan								3	2
Affiliates of Manager \| Third-Party Service Provider
Related Party Transaction [Line Items]
Administrative services expenses incurred							524	$ 385	$ 487
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-4 Term Loan
Related Party Transaction [Line Items]
Total incentive compensation payments						825
Debt securities						$ 33,000	$ 33,000
Percentage of total secured term loans						4.00%	4.00%
Blackstone-Advised Investment Vehicles, or the Funds \| B-3 Term Loan
Related Party Transaction [Line Items]
Total incentive compensation payments	$ 350			$ 350
Debt securities	$ 20,000			$ 20,000
Percentage of total secured term loans	15.00%			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												$ 450.0	246,600	£ 186.0		kr 5.0	€ 250.0	€ 250.0
Total amount of loans acquired												$ 900.0	$ 503,300	£ 379.6		kr 10.2	€ 1,600.0	€ 1,600.0
Participation in facility			49.00%										49.00%	49.00%		49.00%
Blackstone-Advised Investment Vehicles, or the Funds \| Senior Secured Notes
Related Party Transaction [Line Items]
Net book value								$ 400,000
Interest rate								3.75%				3.75%
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
Participation in facility													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												$ 450.0	$ 243,600
Participation in facility													25.00%	25.00%
Blackstone-Advised Investment Vehicles, or the Funds \| Additional Senior Loan Origination Under Marketed Process \| Senior Secured Notes
Related Party Transaction [Line Items]
Net book value								$ 400