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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Income from loans and other investments, net	$ 80,174	$ 78,555	$ 72,473	$ 71,843	$ 71,837	$ 82,817	$ 81,406	$ 77,644	$ 303,045	$ 313,704	$ 258,219
Net income	58,140	57,810	50,613	51,405	53,399	66,392	69,450	57,199	217,968	246,440	211,885
Net income attributable to Blackstone Mortgage Trust, Inc.	$ 57,891	$ 57,722	$ 50,613	$ 51,405	$ 53,375	$ 64,794	$ 63,081	$ 57,047	$ 217,631	$ 238,297	$ 196,829
Net income per share of class A common stock:
Basic and diluted	$ 0.59	$ 0.61	$ 0.53	$ 0.54	$ 0.57	$ 0.69	$ 0.67	$ 0.61	$ 2.27	$ 2.53	$ 2.41