Equity - Additional Information (Detail) - USD ($)				1 Months Ended				12 Months Ended
	Jan. 16, 2018	Dec. 14, 2017	May 09, 2014	Jun. 30, 2015	May 31, 2015	Apr. 30, 2015	May 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	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	12,421,401	1,070	34,780,622
Dividends declared per share of common stock		$ 0.62						$ 2.48	$ 2.48	$ 2.28
Date of dividend paid								Jan. 16, 2018
Date of dividend declared								Dec. 14, 2017
Record date of dividend paid								Dec. 29, 2017
Accumulated other comprehensive loss								$ 29,706,000	$ 56,202,000
Net realized and unrealized gains related to changes in fair value of derivative instruments								30,600,000	51,300,000
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies								60,300,000	107,500,000
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity								42,300,000
Equity interests owned by Blackstone Mortgage Trust, Inc.								36,000,000
Non-controlling interests								$ 6,300,000	$ 0
Subsequent Events [Member]
Class of Stock [Line Items]
Dividends paid	$ 66,900,000
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								9,996,926			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued under dividend reinvestment program								1,401	1,070	597
Number of shares sold during period				11,500,000	280,025	23,000,000		12,420,000		34,780,025
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