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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Income from loans and other investments, net	$ 109,450	$ 114,961	$ 107,129	$ 100,636	$ 109,711	$ 101,916	$ 106,478	$ 106,071	$ 432,177	$ 424,176	$ 396,484
Net (loss) income	84,423	90,769	18,505	(53,283)	79,581	75,394	75,551	76,867	140,414	307,393	285,813
Net income attributable to Blackstone Mortgage Trust, Inc.	$ 83,616	$ 89,860	$ 17,544	$ (53,350)	$ 78,931	$ 74,897	$ 75,174	$ 76,565	$ 137,670	$ 305,567	$ 285,078
Net income per share of class A common stock:
Basic and diluted	$ 0.57	$ 0.61	$ 0.13	$ (0.39)	$ 0.59	$ 0.56	$ 0.59	$ 0.62	$ 0.97	$ 2.35	$ 2.50