Other Expenses - Schedule of General and Administrative Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Professional services	$ 777	$ 648	$ 1,476	$ 1,098
Operating and other costs	554	469	1,112	1,031
Subtotal	10,344	1,117	11,801	2,129
Non-cash and CT Legacy Portfolio compensation expenses
Restricted class A common stock earned	3,303	2,289	6,506	4,029
Subtotal	5,249	13,989	11,353	15,959
Total BXMT expenses	15,698	15,356	23,359	18,554
Expenses of consolidated subsidiaries	15,698	15,356	23,359	18,554
General and Administrative Expense [Member]
Non-cash and CT Legacy Portfolio compensation expenses
Director stock-based compensation	94	94	188	188
CT Legacy Partners [Member]
Non-cash and CT Legacy Portfolio compensation expenses
Management incentive awards plan	1,024	416	2,054	552
Blackstone [Member]
Non-cash and CT Legacy Portfolio compensation expenses
Total BXMT expenses	15,593	15,106	23,154	18,088
Expenses of consolidated subsidiaries	15,593	15,106	23,154	18,088
REIT [Member]
Non-cash and CT Legacy Portfolio compensation expenses
Total BXMT expenses	105	250	205	466
Expenses of consolidated subsidiaries	105	250	205	466
GE Portfolio Acquisition Facility [Member]
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
GE transaction costs	9,013		9,213
CT Opportunity Partners I, LP [Member]
Non-cash and CT Legacy Portfolio compensation expenses
Management incentive awards plan	$ 828	$ 11,190	$ 2,605	$ 11,190