Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended
	Mar. 15, 2021	Nov. 14, 2018	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020	Mar. 25, 2014
Class of Stock [Line Items]
Total stock, shares authorized			500,000,000
Common stock, shares authorized			400,000,000		400,000,000
Dividends paid	$ 91,200
Dividends paid per common stock	$ 0.62
Date of dividend paid			Apr. 15, 2021
Date of dividend declared			Mar. 15, 2021
Record date of dividend paid			Mar. 31, 2021
Accumulated other comprehensive income (loss)			$ 11,284		$ 11,170
Net realized and unrealized gains related to changes in fair value of derivative instruments			39,900		4,800
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies			28,600		6,400
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity			140,500		121,100
Equity interests owned by Blackstone Mortgage Trust, Inc.			119,400		102,900
Non-controlling interests			$ 21,100		$ 18,200
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock			9,991,459			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			515	325
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			515	325
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