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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Income from loans and other investments, net	$ 102,150	$ 105,152	$ 103,746	$ 85,436	$ 80,174	$ 78,555	$ 72,473	$ 71,843	$ 396,484	$ 303,045	$ 313,704
Net income	73,896	78,293	72,507	61,116	58,140	57,810	50,613	51,405	285,813	217,968	246,440
Net income attributable to Blackstone Mortgage Trust, Inc.	$ 73,643	$ 78,165	$ 72,312	$ 60,958	$ 57,891	$ 57,722	$ 50,613	$ 51,405	$ 285,078	$ 217,631	$ 238,297
Net income per share of class A common stock:
Basic and diluted	$ 0.61	$ 0.67	$ 0.66	$ 0.56	$ 0.59	$ 0.61	$ 0.53	$ 0.54	$ 2.50	$ 2.27	$ 2.53