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Mortgage loans on real estate face amount of mortgages foreign currency translation unrealized gain loss.

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percentage of total secured term loans.

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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, kr in Billions	3 Months Ended				9 Months Ended
	Sep. 30, 2021 USD ($) shares	Sep. 30, 2021 GBP (£)	Jun. 30, 2021 EUR (€)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) shares	Dec. 31, 2020 USD ($) shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2021 GBP (£) shares	Sep. 30, 2021 EUR (€) shares	Mar. 31, 2021 USD ($)	Mar. 31, 2021 SEK (kr)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2019 EUR (€)	Sep. 30, 2019 EUR (€)	Jun. 30, 2019 USD ($)
Related Party Transaction [Line Items]
Non-cash expenses					$ 24,184,000		$ 25,978,000
Loan face amount	$ 20,522,560,000				20,522,560,000	$ 16,652,824,000
Unrealized (loss) gain on foreign currency translation Principal Balance					$ (228,637,000)
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares	1,143,320				1,143,320	1,627,890		1,143,320	1,143,320
Senior Term Facility [Member]
Related Party Transaction [Line Items]
Senior term facility \| £								£ 186.0
Percentage of senior loan borrowed	22.00%				22.00%			22.00%	22.00%
Increase of senior term loan facility \| €			€ 100.0
Unrealized (loss) gain on foreign currency translation Principal Balance \| £		£ 379.6
Senior Term Facility [Member] \| Senior Loan Origination Under Marketed Process [Member]
Related Party Transaction [Line Items]
Loan face amount	$ 974,500,000				$ 974,500,000
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement expiration date					Dec. 19, 2021
Management Agreement renewal term, description					the current term of which expires on December 19, 2021, and will be automatically renewed for a one-year term upon such date and each anniversary thereafter unless earlier terminated.
Accrued management and incentive fees payable	19,300,000				$ 19,300,000	$ 19,200,000
Management fees paid to Manager	21,500,000			$ 20,500,000	59,900,000	59,900,000
Expenses reimbursed to Manager	$ 141,000			416,000	$ 325,000		$ 839,000
Manager [Member] \| Class A Common Stock [Member]
Related Party Transaction [Line Items]
Stock Issued During Period, Shares, Issued for Services \| shares							840,696
Management fees paid to Manager							$ 19,300,000
Manager [Member] \| Class A Common Stock [Member] \| Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares	578,914				578,914			578,914	578,914
Non-cash expenses	$ 4,100,000			4,300,000	$ 12,200,000		12,800,000
Manager [Member] \| Class A Common Stock [Member] \| Restricted Class A Common Stock [Member] \| Manager [Member]
Related Party Transaction [Line Items]
Non-cash expenses					18,500,000
Affiliates of Manager [Member] \| Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred	100,000			98,000	291,000		369,000
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	19,300,000				19,300,000	$ 19,200,000
BXMT Advisors Limited Liability Company and Affiliates [Member] \| B Two Term Loan [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments					350,000
BXMT Advisors Limited Liability Company and Affiliates [Member] \| B One Term Loan [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments					950,000
Blackstone-Advised Investment Vehicles, or the Funds [Member] \| B Two Term Loan [Member]
Related Party Transaction [Line Items]
Debt Securities	$ 20,000,000.0			$ 20,000,000.0	$ 20,000,000.0		$ 20,000,000.0
percentage of total secured term loans	5.00%			5.00%	5.00%		5.00%	5.00%	5.00%
Blackstone-Advised Investment Vehicles, or the Funds [Member] \| B One Term Loan [Member]
Related Party Transaction [Line Items]
Debt Securities										$ 65,500,000			$ 65,500,000			$ 65,500,000
percentage of total secured term loans										7.00%	7.00%		7.00%	7.00%		7.00%
Blackstone-Advised Investment Vehicles, or the Funds [Member] \| Senior Term Facility [Member]
Related Party Transaction [Line Items]
Loan face amount									€ 491.0		kr 10.2	$ 421,500,000		€ 1,600.0	€ 1,600.0
Senior term facility	$ 246,600,000		€ 50.0		$ 246,600,000						kr 5.0	$ 140,000,000.0		€ 250.0	€ 250.0
Minority participation in senior term facility	49.00%		10.00%		49.00%			49.00%	49.00%	49.00%	49.00%	33.00%	16.00%	16.00%
Unrealized (loss) gain on foreign currency translation Principal Balance	$ 503,300,000
Blackstone-Advised Investment Vehicles, or the Funds [Member] \| Senior Term Facility [Member] \| Senior Loan Origination Under Marketed Process [Member]
Related Party Transaction [Line Items]
Senior term facility	243,600,000				$ 243,600,000
Blackstone-Advised Investment Vehicles, or the Funds [Member] \| Senior Term Facility [Member] \| Additional Senior Loan Origination Under Marketed Process [Member]
Related Party Transaction [Line Items]
Senior term facility	243,600,000				243,600,000
Unaffiliated Third Parties [Member] \| Senior Term Facility [Member] \| Senior Loan Origination Under Marketed Process [Member]
Related Party Transaction [Line Items]
Senior term facility	$ 487,300,000				$ 487,300,000