Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	12 Months Ended
	Jul. 15, 2020	Nov. 14, 2018	May 31, 2013	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 25, 2014
Class of Stock [Line Items]
Total stock, shares authorized				500,000,000
Common stock, shares authorized				400,000,000	200,000,000
Common stock additional shares authorized charter amendment				200,000,000
Number of shares sold during period			25,875,000	10,842,746	10,535,842	14,568,431
Dividends paid	$ 91,000
Dividends paid per common stock	$ 0.62
Date of dividend paid				Jan. 15, 2021
Date of dividend declared				Dec. 15, 2020
Record date of dividend paid				Dec. 31, 2020
Accumulated other comprehensive income (loss)				$ 11,170	$ (16,233)
Net realized and unrealized gains related to changes in fair value of derivative instruments				4,800	64,500
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies				6,400	80,700
Net proceeds from issuance of class A common stock				278,322	372,337	$ 476,420
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity				121,100	147,300
Equity interests owned by Blackstone Mortgage Trust, Inc.				102,900	125,200
Non-controlling interests				$ 18,200	$ 22,100
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock				9,991,974			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares authorized				400,000,000
Preferred stock, shares authorized				100,000,000
Common stock, shares issued under dividend reinvestment program				2,050	1,214	1,688
Number of shares sold during period				10,840,696	10,534,628	14,566,743
Net proceeds from issuance of class A common stock				$ 297,599	$ 372,341	$ 476,420
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					1,214	1,688
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	7,666,743
Net proceeds from issuance of class A common stock					$ 65,400	$ 254,100