Consolidated Statements of Changes in Equity (USD $) In Thousands	Total	Class A Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income (loss) [Member]	Accumulated Deficit [Member]	Blackstone Mortgage Trust Inc [Member]	Non-Controlling Interests [Member]
Balance at Dec. 31, 2013	$ 756,750	$ 295	$ 1,252,986	$ 798	$ (536,170)	$ 717,909	$ 38,841
Shares of class A common stock issued, net	256,093	99	255,994			256,093
Restricted class A common stock earned	1,740	(1)	1,741			1,740
Deferred directors' compensation	139		139			139
Other comprehensive income (loss)	36			36		36
Net income	13,116				13,065	13,065	51
Dividends declared on common stock	(18,899)				(18,899)	(18,899)
Distributions to non-controlling interests	(575)						(575)
Balance at Mar. 31, 2014	1,008,400	393	1,510,860	834	(542,004)	970,083	38,317
Balance at Dec. 31, 2014	1,500,886	583	2,027,404	(15,024)	(547,592)	1,465,371	35,515
Shares of class A common stock issued, net	2	2				2
Restricted class A common stock earned	3,202		3,202			3,202
Dividends reinvested	3		60		(57)	3
Deferred directors' compensation	94		94			94
Other comprehensive income (loss)	(16,732)			(16,732)		(16,732)
Net income	46,093				35,393	35,393	10,700
Dividends declared on common stock	(30,398)				(30,398)	(30,398)
Distributions to non-controlling interests	(452)						(452)
Balance at Mar. 31, 2015	$ 1,502,698	$ 585	$ 2,030,760	$ (31,756)	$ (542,654)	$ 1,456,935	$ 45,763