Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Net income	$ 217,968	$ 246,440	$ 211,885
Other comprehensive income
Unrealized gain (loss) on foreign currency translation	47,124	(49,686)	(41,616)
Realized and unrealized (loss) gain on derivative financial instruments	(20,628)	26,242	23,882
Other comprehensive income (loss)	26,496	(23,444)	(17,734)
Comprehensive income	244,464	222,996	194,151
Comprehensive income attributable to non-controlling interests	(337)	(8,143)	(15,056)
Comprehensive income attributable to Blackstone Mortgage Trust, Inc.	$ 244,127	$ 214,853	$ 179,095