Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended
	Apr. 15, 2020	Nov. 14, 2018	May 31, 2013	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	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
Plan adoption date				Mar. 25, 2014
Number of shares sold during period			25,875,000	325	1,909,909
Dividends paid	$ 83,900
Dividends paid per common stock	$ 0.62
Date of dividend paid				Apr. 15, 2020
Date of dividend declared				Mar. 13, 2020
Record date of dividend paid				Mar. 31, 2020
Accumulated other comprehensive income (loss)				$ 18,248		$ (16,233)
Net realized and unrealized gains related to changes in fair value of derivative instruments				150,300		64,500
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies				168,500		80,700
Net proceeds from issuance of class A common stock				0	$ 65,377
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity				156,700		147,300
Equity interests owned by Blackstone Mortgage Trust, Inc.				133,200		125,200
Non-controlling interests				$ 23,500		$ 22,100
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock				9,993,699			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares issued under dividend reinvestment program				325	281
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				325	281
Class A Common Stock [Member] | ATM Agreements [Member]
Class of Stock [Line Items]
Aggregate sales price		$ 500,000
Aggregate sales price remaining available				$ 363,800
Class A Common Stock [Member] | BXMT At The Market Agreement [Member]
Class of Stock [Line Items]
Number of shares sold during period					1,909,628
Net proceeds from issuance of class A common stock					$ 65,400