Other Expenses - Schedule of General and Administrative Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Professional services	$ 1,139	$ 862	$ 2,361	$ 1,878
Operating and other costs	830	489	1,339	934
Subtotal	1,969	1,351	3,700	2,812
Non-cash compensation expenses
Restricted class A common stock earned	6,653	5,865	13,504	11,678
Subtotal	6,778	5,959	13,754	11,866
Total BXMT expenses	8,747	7,310	17,454	14,678
Other expenses		63		122
Total general and administrative expenses	8,747	7,373	17,454	14,800
General and Administrative Expense [Member]
Non-cash compensation expenses
Director stock-based compensation	$ 125	$ 94	$ 250	$ 188