Other Expenses - Schedule of General and Administrative Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Income and Expenses [Abstract]
Professional services	$ 1,022	$ 933	$ 3,383	$ 2,811
Operating and other costs	687	489	2,026	1,424
Subtotal	1,709	1,422	5,409	4,235
Non-cash compensation expenses
Restricted class A common stock earned	6,609	5,819	20,113	17,496
Director stock-based compensation	125	125	375	313
Subtotal	6,734	5,944	20,488	17,809
Total BXMT expenses	8,443	7,366	25,897	22,044
Other expenses		53		175
Total general and administrative expenses	$ 8,443	$ 7,419	$ 25,897	$ 22,219