Summary of Quarterly Results of Operations - Summary of Quarterly Results of Operations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 26,837	$ 18,853	$ 6,017	$ 1,456	$ 6,517	$ 6,944	$ 6,763	$ 14,716	$ 21,975	$ 25,642	$ 32,553	$ 36,991
Net income (loss)	9,728	10,526	6,768	(1,597)	122,931	12,900	3,351	140,622	(16,078)	8,256	6,224	253,917	25,424	279,804	252,319
Net income (loss) attributable to Blackstone Mortgage Trust, Inc.	$ 7,079	$ 8,320	$ 2,748	$ (3,115)	$ 105,189	$ 6,999	$ 2,283	$ 66,553	$ (8,320)	$ 13,722	$ (1,845)	$ 254,585	$ 15,032	$ 181,024	$ 258,142
Net income (loss) per share of class A common stock:
Basic	$ 0.24	$ 0.29	$ 0.22	$ (1.03)	$ 42.21	$ 3.02	$ 1.00	$ 29.14	$ (3.66)	$ 6.04	$ (0.81)	$ 113.47	$ 0.81	$ 77.16	$ 113.92
Diluted	$ 0.24	$ 0.29	$ 0.22	$ (1.03)	$ 40.65	$ 2.84	$ 0.93	$ 27.39	$ (3.66)	$ 5.69	$ (0.81)	$ 110.36	$ 0.81	$ 73.13	$ 107.78