Summary of Quarterly Results of Operations - Summary of Quarterly Results of Operations (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Income from loans and other investments, net	$ 82,094	$ 87,032	$ 49,847	$ 39,246	$ 36,812	$ 30,483	$ 26,746	$ 21,582	$ 258,219	$ 115,623	$ 35,147
Net income	65,597	67,778	32,417	46,093	25,338	23,601	38,439	13,116	211,885	100,494	25,424
Net income attributable to Blackstone Mortgage Trust, Inc.	$ 65,264	$ 66,888	$ 29,284	$ 35,393	$ 21,490	$ 22,024	$ 33,466	$ 13,065	$ 196,829	$ 90,045	$ 15,032
Net income per share of class A common stock:
Basic and diluted	$ 0.70	$ 0.72	$ 0.36	$ 0.60	$ 0.37	$ 0.45	$ 0.70	$ 0.34	$ 2.41	$ 1.86	$ 0.81