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Repurchase Agreements - Additional Information (Detail) - USD ($)
Jun. 29, 2018
Sep. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]      
Additional financing capacity secured by investments in real estate-related securities   $ 2,029,334,000 $ 915,742,000
Commercial Mortgage Backed Securities [Member]      
Debt Instrument [Line Items]      
Additional financing capacity secured by investments in real estate-related securities   $ 2,029,334,000 $ 915,742,000
Barclays MRA [Member]      
Debt Instrument [Line Items]      
Maximum facility size $ 750,000,000    
Repurchase agreements initial maturity date Sep. 29, 2021