Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands			12 Months Ended
	Dec. 15, 2021 USD ($) $ / shares	Nov. 14, 2018 USD ($) agreement	Dec. 31, 2021 USD ($) shares	Dec. 31, 2020 USD ($) shares	Dec. 31, 2019 USD ($) shares	Mar. 25, 2014 shares
Class of Stock [Line Items]
Shares authorized (in shares) | shares			500,000,000
Common stock, shares authorized (in shares) | shares			400,000,000
Preferred stock, shares authorized (in shares) | shares			100,000,000
Common stock, shares issued under dividend reinvestment program (in shares) | shares			2,184	2,050	1,214
Dividends paid per common stock (in USD per share) | $ / shares	$ 0.62
Dividends paid	$ 104,300
Accumulated other comprehensive income			$ 8,308	$ 11,170
Net realized and unrealized gains related to changes in fair value of derivative instruments			86,400	4,800
Cumulative unrealized currency translation adjustment on assets and liabilities denominated in foreign currencies			78,100	6,400
Joint Venture | Multifamily
Class of Stock [Line Items]
Total equity			203,500	121,100
Equity interests owned by Blackstone Mortgage Trust, Inc.			173,000	102,900
Non-controlling interests			$ 30,500	$ 18,200
Dividend Reinvestment and Direct Stock Purchase Plan
Class of Stock [Line Items]
Common shares reserved for issuance (in shares) | shares			9,989,790			10,000,000
ATM Agreement
Class of Stock [Line Items]
Number of equity distribution agreements | agreement		6
Aggregate sales price		$ 500,000
Number of additional ATM Agreements | agreement		1
Number of shares sold (in shares) | shares			296,901		1,909,628
Shares sold			$ 9,900		$ 65,400
Aggregate sales price remaining available			$ 353,800