Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended		3 Months Ended		6 Months Ended
	Jul. 15, 2020	Nov. 14, 2018	May 31, 2020	May 31, 2013	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Mar. 25, 2014
Class of Stock [Line Items]
Total stock, shares authorized					500,000,000		500,000,000
Common stock, shares authorized					400,000,000		400,000,000		200,000,000
Number of shares sold during period				25,875,000			10,841,667	10,535,181
Date of dividend paid							Jul. 15, 2020
Date of dividend declared							Jun. 15, 2020
Record date of dividend paid							Jun. 30, 2020
Accumulated other comprehensive income (loss)					$ 8,925		$ 8,925		$ (16,233)
Net realized and unrealized gains related to changes in fair value of derivative instruments					128,900		128,900		64,500
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies					137,800		137,800		80,700
Net proceeds from issuance of class A common stock							278,322	$ 372,329
Subsequent Event [Member]
Class of Stock [Line Items]
Dividends paid	$ 90,600
Dividends paid per common stock	$ 0.62
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity					140,100		140,100		147,300
Equity interests owned by Blackstone Mortgage Trust, Inc.					119,100		119,100		125,200
Non-controlling interests					$ 21,000		$ 21,000		$ 22,100
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock					9,993,053		9,993,053			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares authorized					100,000,000		100,000,000
Common stock, shares issued under dividend reinvestment program					646	272	971	553
Number of shares sold during period			840,696				10,000,000
Net proceeds from issuance of class A common stock			$ 19,277				$ 278,322
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							971	553
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		$ 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