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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Income from loans and other investments, net	$ 71,837	$ 82,817	$ 81,406	$ 77,644	$ 82,094	$ 87,032	$ 49,847	$ 39,246	$ 313,704	$ 258,219	$ 115,623
Net income	53,399	66,392	69,450	57,199	65,597	67,778	32,417	46,093	246,440	211,885	100,494
Net income attributable to Blackstone Mortgage Trust, Inc.	$ 53,375	$ 64,794	$ 63,081	$ 57,047	$ 65,264	$ 66,888	$ 29,284	$ 35,393	$ 238,297	$ 196,829	$ 90,045
Net income per share of class A common stock:
Basic and diluted	$ 0.57	$ 0.69	$ 0.67	$ 0.61	$ 0.70	$ 0.72	$ 0.36	$ 0.60	$ 2.53	$ 2.41	$ 1.86