Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 16, 2018	Jun. 15, 2018	May 09, 2014	May 31, 2013	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	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			3,252,325	543
Net proceeds from issuance of class A common stock							$ 102,496	$ 16
Dividends declared per share of common stock		$ 0.62			$ 0.62	$ 0.62	$ 1.24	$ 1.24
Date of dividend paid							Jul. 16, 2018
Date of dividend declared							Jun. 15, 2018
Record date of dividend paid							Jun. 29, 2018
Accumulated other comprehensive loss					$ (32,136)		$ (32,136)		$ (29,706)
Net realized and unrealized gains related to changes in fair value of derivative instruments					52,600		52,600		30,600
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies					84,700		84,700		60,300
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity					34,500		34,500		42,300
Equity interests owned by Blackstone Mortgage Trust, Inc.					29,300		29,300		36,000
Non-controlling interests					$ 5,200		$ 5,200		$ 6,300
Subsequent Events [Member]
Class of Stock [Line Items]
Dividends paid	$ 69,100
Dividends paid per common stock	$ 0.62
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock					9,996,050		9,996,050			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued under dividend reinvestment program					421	273	876	543
Number of shares sold during period					3,251,449
Net proceeds from issuance of class A common stock					$ 102,496
ATM Agreements [Member]
Class of Stock [Line Items]
Aggregate sales price			$ 200,000
Number of shares sold during period							3,251,449	0
Net proceeds from issuance of class A common stock							$ 102,500
Aggregate sales price remaining available					$ 84,100		$ 84,100