Transactions with Related Parties - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2017 USD ($) SecurityLoan shares	Mar. 31, 2016 USD ($) shares	Dec. 31, 2016 USD ($) SecurityLoan shares
Related Party Transaction [Line Items]
Accrued management and incentive fees payable	$ 12,921,000		$ 12,798,000
Common stock value	948,000		$ 945,000
Non-cash expenses	$ 5,907,000	$ 4,852,000
Number of Loans | SecurityLoan	103		107
Restricted Class A Common Stock [Member]
Related Party Transaction [Line Items]
Shares held | shares	1,476,111		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	$ 12,900,000		$ 12,800,000
Management fees paid to Manager	12,800,000	$ 14,400,000
Expenses reimbursed to Manager	$ 93,000	320,000
Number of Loans | SecurityLoan	2
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	719,903
Common stock value	$ 20,900,000
Non-cash expenses	$ 2,900,000	2,100,000
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	$ 224,000
Affiliates of Manager [Member] | Third-Party Service Provider [Member]
Related Party Transaction [Line Items]
Administrative services expenses incurred	$ 88,000	$ 90,000