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, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	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, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 58,258	$ 50,386	$ 42,466	$ 33,656	$ 26,837	$ 18,853	$ 6,017	$ 1,456	$ 6,517	$ 6,944	$ 6,763	$ 14,716
Net (loss) income	25,338	23,601	38,439	13,116	9,728	10,526	6,768	(1,597)	122,931	12,900	3,351	140,622	100,494	25,424	279,804
Net (loss) income attributable to Blackstone Mortgage Trust, Inc.	$ 21,490	$ 22,024	$ 33,466	$ 13,065	$ 7,079	$ 8,320	$ 2,748	$ (3,115)	$ 105,189	$ 6,999	$ 2,283	$ 66,553	$ 90,045	$ 15,032	$ 181,024
Net (loss) income per share of class A common stock:
Basic	$ 0.37	$ 0.45	$ 0.70	$ 0.34	$ 0.24	$ 0.29	$ 0.22	$ (1.03)	$ 42.21	$ 3.02	$ 1.00	$ 29.14	$ 1.86	$ 0.81	$ 77.16
Diluted	$ 0.37	$ 0.45	$ 0.70	$ 0.34	$ 0.24	$ 0.29	$ 0.22	$ (1.03)	$ 40.65	$ 2.84	$ 0.93	$ 27.39	$ 1.86	$ 0.81	$ 73.13