Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended	12 Months Ended
	Jan. 15, 2019	Dec. 13, 2018	Nov. 14, 2018	May 31, 2013	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 25, 2014
Class of Stock [Line Items]
Total stock, shares authorized					300,000,000
Common stock, shares authorized					200,000,000	200,000,000
Preferred stock, shares authorized					100,000,000
Preferred stock issued					0
Preferred stock outstanding					0
Plan adoption date					Mar. 25, 2014
Number of shares sold during period				25,875,000	14,568,431	12,421,401	1,070
Dividends paid	$ 76,500
Dividends declared per share of common stock		$ 0.62			$ 2.48	$ 2.48	$ 2.48
Dividends paid per common stock	$ 0.62
Date of dividend paid					Jan. 15, 2019
Date of dividend declared					Dec. 13, 2018
Record date of dividend paid					Dec. 31, 2018
Accumulated other comprehensive loss					$ (34,222)	$ (29,706)
Net realized and unrealized gains related to changes in fair value of derivative instruments					70,400	30,600
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies					104,600	60,300
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity					69,900	42,300
Equity interests owned by Blackstone Mortgage Trust, Inc.					59,400	36,000
Non-controlling interests					$ 10,500	$ 6,300
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock					9,995,238			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued under dividend reinvestment program					1,688	1,401	1,070
Number of shares sold during period					14,566,743
Class A Common Stock [Member] | ATM Agreements [Member]
Class of Stock [Line Items]
Aggregate sales price			$ 500,000
Number of shares sold during period					7,666,743	12,420,000
Aggregate sales price remaining available					$ 429,900