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Amount of expense for administrative services provided to the limited liability company (LLC) or limited partnership (LP) by the managing member or general partner, affiliate of managing member or general partner, or affiliate of LLC or LP, for example, but not limited to, salaries, rent, or overhead costs.

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Amount, after allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), investment in debt security measured at amortized cost (held-to-maturity), and investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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Amount paid to managing member or general partner for management of the day-to-day business functions of the limited liability company (LLC) or limited partnership (LP).

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Professional and contract service expense includes cost reimbursements for support services related to contracted projects, outsourced management, technical and staff support.

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Transactions with Related Parties - Additional Information (Detail) € in Millions, £ in Millions, $ in Millions, kr in Billions	3 Months Ended							12 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2021 EUR (€)	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Jun. 30, 2019 USD ($)	Dec. 31, 2021 USD ($) loan shares	Dec. 31, 2020 USD ($) loan shares	Dec. 31, 2019 USD ($) loan shares	Dec. 31, 2021 AUD ($) shares	Sep. 30, 2021 USD ($)	Sep. 30, 2021 GBP (£)	Jun. 30, 2021 EUR (€)	Mar. 31, 2021 SEK (kr)	Dec. 31, 2019 EUR (€) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2019 EUR (€)
Related Party Transaction [Line Items]
Shares issued for management and incentive fees (in shares) \| shares					840,696
Non-cash expenses								$ 31,647,000	$ 34,532,000	$ 30,656,000
Loans acquired								21,878,338,000	16,399,166,000
Unrealized gain (loss) on foreign currency translation								$ (299,318,000)	$ 342,160,000
Senior Term Facility
Related Party Transaction [Line Items]
Loans acquired \| £													£ 186.0
Participation in facility						16.00%				16.00%						16.00%	16.00%	16.00%
Total amount of loans acquired \| £													£ 379.6
Additional interest in loans \| €		€ 100.0
Percentage ownership on loan	22.00%													22.00%
B One Term Loan
Related Party Transaction [Line Items]
Debt securities						$ 60,000,000	$ 60,000,000			$ 60,000,000
Percentage of total secured term loans						8.00%	8.00%			8.00%						8.00%
Senior Loan Origination Under Marketed Process \| Senior Term Facility
Related Party Transaction [Line Items]
Total amount of loans acquired												$ 974,500,000
BXMT Advisors Limited Liability Company and Affiliates \| B One Term Loan
Related Party Transaction [Line Items]
Total incentive compensation payments						$ 750,000	$ 750,000
Restricted Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) \| shares						1,698,582		1,706,121	1,627,890	1,698,582	1,706,121					1,698,582
Restricted shares, vesting period								3 years
BXMT Advisors Limited Liability Company and Affiliates
Related Party Transaction [Line Items]
Accrued management and incentive fees payable								$ 28,400,000	$ 19,200,000
BXMT Advisors Limited Liability Company and Affiliates \| B Two Term Loan
Related Party Transaction [Line Items]
Total incentive compensation payments	$ 350,000			$ 350,000
Manager
Related Party Transaction [Line Items]
Accrued management and incentive fees payable								28,400,000	19,200,000
Management fees paid to Manager								79,300,000	$ 78,900,000	$ 76,900,000
Shares issued for management and incentive fees (in shares) \| shares									840,696
Expenses reimbursed to Manager								$ 601,000	$ 1,000,000	1,100,000
Manager \| Class A Common Stock
Related Party Transaction [Line Items]
Management fees paid to Manager			$ 19,300,000
Shares issued for management and incentive fees (in shares) \| shares					840,696			840,696
Manager \| Class A Common Stock \| Restricted Class A Common Stock
Related Party Transaction [Line Items]
Shares held (in shares) \| shares								1,100,000			1,100,000
Non-cash expenses								$ 15,300,000	$ 17,000,000	$ 15,100,000
Manager \| Class A Common Stock \| Restricted Class A Common Stock \| Manager
Related Party Transaction [Line Items]
Non-cash expenses								$ 33,400,000
Restricted shares, vesting period								3 years
Affiliates of Manager
Related Party Transaction [Line Items]
Number of loans originated \| loan								3	2	9
Affiliates of Manager \| Third-Party Service Provider
Related Party Transaction [Line Items]
Administrative services expenses incurred								$ 385,000	$ 487,000	$ 440,000
Blackstone-Advised Investment Vehicles, or the Funds \| Senior Term Facility
Related Party Transaction [Line Items]
Loans acquired					$ 140,000,000.0						$ 450.0	$ 246,600,000		€ 50.0	kr 5.0	€ 250.0	$ 214,300,000	€ 250.0
Participation in facility	10.00%				33.00%							49.00%	49.00%	10.00%	49.00%		49.00%	49.00%
Total amount of loans acquired					$ 421,500,000						$ 900.0	$ 503,000,000.0		€ 491.0	kr 10.2	€ 1,600.0	$ 437,400,000	€ 1,600.0
Blackstone-Advised Investment Vehicles, or the Funds \| Senior Secured Notes
Related Party Transaction [Line Items]
Loans acquired								$ 400,000,000.0
Stated interest rate								3.75%			3.75%
Blackstone-Advised Investment Vehicles, or the Funds \| B Two Term Loan
Related Party Transaction [Line Items]
Debt securities	$ 20,000,000.0			$ 20,000,000.0
Percentage of total secured term loans	5.00%			5.00%										5.00%
Blackstone-Advised Investment Vehicles, or the Funds \| Senior Loan Origination Under Marketed Process \| Senior Term Facility
Related Party Transaction [Line Items]
Loans acquired												243,600,000
Blackstone-Advised Investment Vehicles, or the Funds \| Additional Senior Loan Origination Under Marketed Process \| Senior Term Facility
Related Party Transaction [Line Items]
Loans acquired											$ 450.0	243,600,000
Blackstone-Advised Investment Vehicles, or the Funds \| Additional Senior Loan Origination Under Marketed Process \| Senior Secured Notes
Related Party Transaction [Line Items]
Loans acquired								$ 400,000
Unaffiliated Third Parties \| Senior Loan Origination Under Marketed Process \| Senior Term Facility
Related Party Transaction [Line Items]
Loans acquired												$ 487,300,000