Other Expenses - Schedule of General and Administrative Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Professional services	$ 898	$ 635	$ 2,375	$ 1,733
Operating and other costs	671	451	1,783	1,482
Subtotal	1,939	1,086	13,741	3,215
Non-cash and CT Legacy Portfolio compensation expenses
Restricted class A common stock earned	3,095	1,525	9,601	5,554
Subtotal	3,286	2,077	14,638	18,035
Total BXMT expenses	5,295	3,368	28,655	21,920
Total BXMT expenses	5,295	3,368	28,655	21,920
General and Administrative Expense [Member]
Non-cash and CT Legacy Portfolio compensation expenses
Director stock-based compensation	94	94	281	281
CT Legacy Partners [Member]
Non-cash and CT Legacy Portfolio compensation expenses
Management incentive awards plan	89	458	2,151	1,010
Blackstone [Member]
Non-cash and CT Legacy Portfolio compensation expenses
Total BXMT expenses	5,225	3,163	28,379	21,250
Total BXMT expenses	5,225	3,163	28,379	21,250
REIT [Member]
Non-cash and CT Legacy Portfolio compensation expenses
Total BXMT expenses	70	205	276	670
Total BXMT expenses	70	$ 205	276	670
General Electrical Capital Corporation [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
GE transaction costs	370		9,583
CT Opportunity Partners I, LP [Member]
Non-cash and CT Legacy Portfolio compensation expenses
Management incentive awards plan	$ 8		$ 2,605	$ 11,190