Equity - Additional Information (Detail) - USD ($)				1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 15, 2019	Jun. 14, 2019	Nov. 14, 2018	May 31, 2013	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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			10,535,181	3,252,325
Dividends paid	$ 83,300,000
Dividends paid per common stock		$ 0.62
Date of dividend paid							Jul. 15, 2019
Date of dividend declared							Jun. 14, 2019
Record date of dividend paid							Jun. 28, 2019
Accumulated other comprehensive loss					$ (29,420,000)		$ (29,420,000)		$ (34,222,000)
Net realized and unrealized gains related to changes in fair value of derivative instruments					79,400,000		79,400,000		70,400,000
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies					108,800,000		108,800,000		104,600,000
Net proceeds from issuance of class A common stock							372,329,000	$ 102,496,000
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity					193,700,000		193,700,000		69,900,000
Equity interests owned by Blackstone Mortgage Trust, Inc.					164,600,000		164,600,000		59,400,000
Non-controlling interests					$ 29,100,000		$ 29,100,000		$ 10,500,000
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock					9,994,685		9,994,685			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued under dividend reinvestment program							553	876
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					272	421	553	876
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	3,251,449
Aggregate sales price remaining available					$ 363,800,000		$ 363,800,000
Net proceeds from issuance of class A common stock							$ 65,400,000	$ 102,500,000