Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				1 Months Ended	3 Months Ended
	Apr. 16, 2018	Mar. 13, 2018	May 09, 2014	May 31, 2013	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	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	455	270
Dividends declared per share of common stock		$ 0.62			$ 0.62	$ 0.62
Date of dividend paid					Apr. 16, 2018
Date of dividend declared					Mar. 13, 2018
Record date of dividend paid					Mar. 30, 2018
Accumulated other comprehensive loss					$ 21,903		$ 29,706
Net realized and unrealized gains related to changes in fair value of derivative instruments					27,700		30,600
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies					49,600		60,300
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity					35,300		42,300
Equity interests owned by Blackstone Mortgage Trust, Inc.					30,000		36,000
Non-controlling interests					$ 5,300		$ 6,300
Subsequent Events [Member]
Class of Stock [Line Items]
Dividends paid	$ 67,100
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					9,996,471			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued under dividend reinvestment program					455	270
ATM Agreements [Member]
Class of Stock [Line Items]
Aggregate sales price			$ 200,000
Number of shares sold during period					0	0
Aggregate sales price remaining available					$ 188,600