Transactions with Related Parties - Additional Information (Detail)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) shares	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) SecurityLoan shares	Jun. 30, 2016 USD ($) shares	Dec. 31, 2016 USD ($) SecurityLoan shares
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	$ 14,393,000		$ 14,393,000		$ 12,798,000
Common stock value	948,000		948,000		$ 945,000
Non-cash expenses			$ 11,866,000	$ 10,315,000
Number of Loans | SecurityLoan			102		107
Fully subscribed offering costs	474,600,000		$ 474,600,000
Securitized debt obligations issued, aggregate amount	$ 72,900,000		$ 72,900,000
Maximum [Member]
Related Party Transaction [Line Items]
Participation percentage in individual tranches, represented by investment vehicles			49.00%
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares	1,374,009		1,374,009		1,309,995
Restricted shares, vesting period			3 years
Restricted Class A Common Stock [Member] | Class A Common Stock [Member]
Related Party Transaction [Line Items]
Common stock, shares issued | shares			0	0
Manager [Member]
Related Party Transaction [Line Items]
Management Agreement expiration date			Dec. 19, 2017
Management Agreement renewal term, description			The current term of which expires on December 19, 2017, and will be automatically renewed for a one-year term each anniversary thereafter unless earlier terminated.
Management Agreement renewal term, period			1 year
Accrued management and incentive fees payable	$ 14,400,000		$ 14,400,000		$ 12,800,000
Management fees paid to Manager	12,900,000	$ 13,600,000	25,700,000	$ 28,000,000
Expenses reimbursed to Manager	$ 174,000	59,000	$ 266,000	380,000
Number of Loans | SecurityLoan			3
Expenses incurred on insurance agency transaction of affiliate with loan borrower			$ 0	0
Income received on insurance agency transaction of affiliate with loan borrower			$ 0	0
Manager [Member] | Class A Common Stock [Member] | Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares	663,846		663,846
Common stock value	$ 19,300,000		$ 19,300,000
Non-cash expenses	2,900,000	2,500,000	$ 5,800,000	4,600,000
Manager [Member] | Class A Common Stock [Member] | Restricted Class A Common Stock [Member] | Stock Incentive Current Plan [Member]
Related Party Transaction [Line Items]
Restricted shares, vesting period			3 years
BXMT Advisors Limited Liability Company and Affiliates [Member]
Related Party Transaction [Line Items]
Payments of preferred distributions to affiliate of our Manager		121,000		345,000
Affiliates of Manager [Member] | Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred	$ 79,000	$ 80,000	$ 167,000	$ 170,000