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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Income from loans and other investments, net	$ 109,711	$ 101,916	$ 106,478	$ 106,071	$ 102,150	$ 105,152	$ 103,746	$ 85,436	$ 424,176	$ 396,484	$ 303,045
Net income	79,581	75,394	75,551	76,867	73,896	78,293	72,507	61,116	307,393	285,813	217,968
Net income attributable to Blackstone Mortgage Trust, Inc.	$ 78,931	$ 74,897	$ 75,174	$ 76,565	$ 73,643	$ 78,165	$ 72,312	$ 60,958	$ 305,567	$ 285,078	$ 217,631
Net income per share of class A common stock:
Basic and diluted	$ 0.59	$ 0.56	$ 0.59	$ 0.62	$ 0.61	$ 0.67	$ 0.66	$ 0.56	$ 2.35	$ 2.50	$ 2.27