Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended		6 Months Ended
	Jun. 15, 2021	Nov. 14, 2018	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	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		400,000,000
Dividends paid	$ 91,100
Dividends paid per common stock	$ 0.62
Date of dividend paid					Jul. 15, 2021
Date of dividend declared					Jun. 15, 2021
Record date of dividend paid					Jun. 30, 2021
Accumulated other comprehensive income (loss)			$ 10,743		$ 10,743		$ 11,170
Net realized and unrealized gains related to changes in fair value of derivative instruments			23,700		23,700		4,800
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies			13,000		13,000		6,400
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity			173,300		173,300		121,100
Equity interests owned by Blackstone Mortgage Trust, Inc.			147,300		147,300		102,900
Non-controlling interests			$ 26,000		$ 26,000		$ 18,200
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock			9,991,025		9,991,025			10,000,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares authorized			400,000,000		400,000,000
Preferred stock, shares authorized			100,000,000		100,000,000
Common stock, shares issued under dividend reinvestment program					949	971
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			434	646	949	971
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