Equity - Additional Information (Detail) - USD ($)				1 Months Ended	3 Months Ended		6 Months Ended		9 Months Ended
	Oct. 13, 2017	Sep. 15, 2017	May 09, 2014	May 31, 2013	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Mar. 25, 2014
Class of Stock [Line Items]
Total stock, shares authorized					300,000,000		300,000,000		300,000,000
Common stock, shares authorized					200,000,000		200,000,000		200,000,000		200,000,000
Preferred stock, shares authorized					100,000,000		100,000,000		100,000,000
Preferred stock issued					0		0		0
Preferred stock outstanding					0		0		0
Plan adoption date									Mar. 25, 2014
Common stock, shares issued under dividend reinvestment program					428	262			971	812
Number of shares sold during period				25,875,000					971	812
Dividends declared per share of common stock		$ 0.62			$ 0.62	$ 0.62			$ 1.86	$ 1.86
Date of dividend paid									Jul. 14, 2017
Date of dividend declared									Jun. 13, 2017
Record date of dividend paid									Jun. 30, 2017
Accumulated other comprehensive loss					$ 32,362,000		$ 32,362,000		$ 32,362,000		$ 56,202,000
Net realized and unrealized gains related to changes in fair value of derivative instruments					31,100,000		31,100,000		31,100,000		51,300,000
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies					63,500,000		63,500,000		63,500,000		107,500,000
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity					41,200,000		41,200,000		41,200,000
Equity interests owned by Blackstone Mortgage Trust, Inc.					35,000,000		35,000,000		35,000,000
Non-controlling interests					6,200,000		$ 6,200,000		6,200,000		$ 0
Subsequent Events [Member]
Class of Stock [Line Items]
Dividends paid	$ 58,800,000
Dividends paid per common stock	$ 0.62
ATM Agreements [Member]
Class of Stock [Line Items]
Aggregate sales price			$ 200,000,000
Number of shares sold during period							0	0
Aggregate sales price remaining available					$ 188,600,000		$ 188,600,000		$ 188,600,000
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock					9,997,356		9,997,356		9,997,356			10,000,000