Equity - Additional Information (Detail) - USD ($)				1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 15, 2019	Jun. 14, 2019	Nov. 14, 2018	May 31, 2013	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Mar. 25, 2014
Class of Stock [Line Items]
Total stock, shares authorized					300,000,000		300,000,000
Common stock, shares authorized					200,000,000		200,000,000		200,000,000
Preferred stock, shares authorized					100,000,000		100,000,000
Preferred stock issued					0		0
Preferred stock outstanding					0		0
Plan adoption date							Mar. 25, 2014
Number of shares sold during period				25,875,000			10,535,507	11,484,414
Dividends paid	$ 83,300,000
Dividends paid per common stock		$ 0.62
Date of dividend paid							Oct. 15, 2019
Date of dividend declared							Sep. 13, 2019
Record date of dividend paid							Sep. 30, 2019
Accumulated other comprehensive loss					$ (33,394,000)		$ (33,394,000)		$ (34,222,000)
Net realized and unrealized gains related to changes in fair value of derivative instruments					115,400,000		115,400,000		70,400,000
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies					148,800,000		148,800,000		104,600,000
Net proceeds from issuance of class A common stock							372,329,000	$ 369,787,000
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity					118,800,000		118,800,000		69,900,000
Equity interests owned by Blackstone Mortgage Trust, Inc.					101,000,000.0		101,000,000.0		59,400,000
Non-controlling interests					$ 17,800,000		$ 17,800,000		$ 10,500,000
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock					9,994,359		9,994,359			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued under dividend reinvestment program							879	1,279
Number of shares sold during period							8,625,000
Net proceeds from issuance of class A common stock							$ 306,952
Class A Common Stock [Member] | Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Common stock, shares issued under dividend reinvestment program					326	403	879	1,279
Class A Common Stock [Member] | ATM Agreements [Member]
Class of Stock [Line Items]
Aggregate sales price			$ 500,000,000.0
Number of shares sold during period							1,909,628	4,583,135
Aggregate sales price remaining available					$ 363,800,000		$ 363,800,000
Net proceeds from issuance of class A common stock							$ 65,400,000	$ 147,500,000