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

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The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity.

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Transactions with Related Parties - Additional Information (Detail) € in Millions, £ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) SecurityLoan	Dec. 31, 2018 USD ($) SecurityLoan shares	Dec. 31, 2017 USD ($) SecurityLoan shares	Dec. 31, 2019 GBP (£) shares	Dec. 31, 2019 EUR (€) shares	Dec. 31, 2019 USD ($) shares	Sep. 30, 2019 EUR (€)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 EUR (€)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 GBP (£)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 EUR (€)	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)
Related Party Transaction [Line Items]
Common stock value				$ 1,234,000				$ 1,350,000
Non-cash expenses			$ 30,656,000	$ 28,154,000	$ 24,031,000
Number of Loans \| SecurityLoan			128	125
Loan face amount								16,277,343,000
2018 Single Asset Securitization [Member]
Related Party Transaction [Line Items]
Fully subscribed offering costs				$ 1,000,000,000.0				930,000,000.0		$ 1,000,000,000.0				$ 1,000,000,000.0
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement expiration date			Dec. 19, 2020
Management Agreement renewal term, description			the current term of which expires on December 19, 2020, and will be automatically renewed for a one-year term upon such date and each anniversary thereafter unless earlier terminated.
Secured Term Loan [Member]
Related Party Transaction [Line Items]
Debt Securities								$ 60,000,000.0				$ 60,000,000.0
percentage of total secured term loans						8.00%	8.00%	8.00%			8.00%	8.00%
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares				1,614,907	1,484,175	1,698,582	1,698,582	1,698,582
Restricted shares, vesting period			3 years
Senior Term Facility [Member]
Related Party Transaction [Line Items]
Loan face amount						£ 303.5	€ 1,600.0		€ 1,600.0				£ 490.0			€ 7,300.0
Senior term facility						£ 148.7	€ 250.0		€ 250.0				£ 240.1			€ 1,000.0
Minority participation in senior term facility				49.00%		16.00%	16.00%	16.00%	16.00%	16.00%			49.00%			14.00%	14.00%
Senior Term Facility [Member] \| Loan Purchase Commitments [Member]
Related Party Transaction [Line Items]
Loan face amount \| €							€ 391.3
Senior term facility \| €											€ 191.8
Minority participation in senior term facility						49.00%	49.00%	49.00%
Unaffiliated Third Party [Member]
Related Party Transaction [Line Items]
Loan face amount										$ 437,400,000
Senior term facility										$ 214,300,000
Minority participation in senior term facility									49.00%	49.00%
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement renewal term, period			1 year
Accrued management and incentive fees payable				$ 18,600,000				$ 20,200,000
Management fees paid to Manager			$ 76,900,000	70,500,000
Expenses reimbursed to Manager			$ 1,100,000	$ 836,000
Number of Loans \| SecurityLoan			4	9	6
Expenses incurred on insurance agency transaction of affiliate with loan borrower			$ 0	$ 0	$ 0
Manager [Member] \| Unaffiliated Third Party [Member] \| Senior Loan [Member]
Related Party Transaction [Line Items]
Loan face amount															$ 1,000,000,000.0
Manager [Member] \| Class A Common Stock [Member] \| Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held \| shares						1,017,141	1,017,141	1,017,141
Common stock value								$ 34,600,000
Non-cash expenses			15,100,000	13,500,000	11,700,000
Manager [Member] \| Class A Common Stock [Member] \| Unaffiliated Third Party [Member] \| Partially Owned Properties [Member] \| Senior Loan [Member]
Related Party Transaction [Line Items]
Ownership interest in joint venture															50.00%
Affiliates of Manager [Member] \| Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred			$ 440,000	$ 450,000	411,000
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Management fees paid to Manager					53,400,000
Expenses reimbursed to Manager					$ 621,000
Fully subscribed offering costs																		$ 474,600,000
Securitized debt obligations issued, aggregate amount										$ 116,100,000								$ 72,900,000
BXMT Advisors Limited Liability Company and Affiliates [Member] \| Senior Loan [Member]
Related Party Transaction [Line Items]
Loan face amount																	$ 330,000,000.0
BXMT Advisors Limited Liability Company and Affiliates [Member] \| Secured Term Loan [Member] \| Manager [Member]
Related Party Transaction [Line Items]
Total incentive compensation payments	$ 750,000	$ 750,000
Blackstone-Advised Investment Vehicles, or the Funds [Member] \| Senior Term Facility [Member]
Related Party Transaction [Line Items]
Loan face amount \| €							€ 388.0
Senior term facility \| €							€ 193.6
Minority participation in senior term facility						49.00%	49.00%	49.00%
Ownership interest in joint venture																51.00%	51.00%