Transactions with Related Parties - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) SecurityLoan shares	Sep. 30, 2016 USD ($) SecurityLoan shares	Dec. 31, 2016 USD ($) SecurityLoan shares	Jun. 30, 2017 USD ($)
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	$ 13,243,000		$ 13,243,000		$ 12,798,000
Common stock value	$ 948,000		948,000		$ 945,000
Non-cash expenses			$ 17,809,000	$ 16,517,000
Number of Loans | SecurityLoan			111		107
Fully subscribed offering costs						$ 474,600,000
Securitized debt obligations issued, aggregate amount						$ 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	1,261,232		1,261,232		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	$ 13,200,000		$ 13,200,000		$ 12,800,000
Management fees paid to Manager	14,400,000	$ 15,800,000	40,100,000	$ 43,800,000
Expenses reimbursed to Manager	$ 59,000	82,000	$ 325,000	$ 462,000
Number of Loans | SecurityLoan			5	1
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	607,789		607,789
Common stock value	$ 17,800,000		$ 17,800,000
Non-cash expenses	2,900,000	2,500,000	$ 8,700,000	7,100,000
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		146,000		491,000
Affiliates of Manager [Member] | Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred	$ 87,000	$ 112,000	$ 254,000	$ 282,000