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Summary Information Concerning Indebtedness (Parenthetical) (Detail) - USD ($)
Dec. 31, 2019
Dec. 31, 2018
May 01, 2017
Debt Instrument [Line Items]      
Outstanding Principal $ 991,178,000 $ 991,415,000  
Unsecured Credit Facility      
Debt Instrument [Line Items]      
Credit facility borrowing capacity 350,000,000   $ 300,000,000
Outstanding Principal $ 186,302,000 [1] 155,743,000 [2]  
Secured Credit Facility      
Debt Instrument [Line Items]      
Credit facility borrowing capacity   300,000,000  
Outstanding Principal   $ 155,743,000  
[1] The unsecured credit facility total capacity is $350,000, of which $186,302 was outstanding as of December 31, 2019.
[2] The secured credit facility total capacity was $300,000, of which $155,743 was outstanding as of December 31, 2018.