Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		9 Months Ended
	Nov. 14, 2018	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 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				$ 97,300
Dividends paid per common stock				$ 0.62
Date of dividend paid				Oct. 15, 2021
Date of dividend declared				Sep. 15, 2021
Record date of dividend paid				Sep. 30, 2021
Accumulated other comprehensive income (loss)		$ 9,874		$ 9,874		$ 11,170
Net realized and unrealized gains related to changes in fair value of derivative instruments		69,900		69,900		4,800
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies		60,000		60,000		6,400
Multifamily [Member] | Joint Venture [Member]
Class of Stock [Line Items]
Total equity		226,700		226,700		121,100
Equity interests owned by Blackstone Mortgage Trust, Inc.		192,700		192,700		102,900
Non-controlling interests		$ 34,000		$ 34,000		$ 18,200
Dividend Reinvestment and Direct Stock Purchase Plan [Member]
Class of Stock [Line Items]
Reserved for issuance of class A common stock		9,990,545		9,990,545			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				1,429	1,599
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		480	628	1,429	1,599
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