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Summary Information Concerning Indebtedness (Parenthetical) (Detail) - USD ($)
Dec. 31, 2018
Dec. 31, 2017
May 01, 2017
Debt Instrument [Line Items]      
Outstanding Principal $ 991,415,000 $ 784,640,000  
Unsecured Credit Facility      
Debt Instrument [Line Items]      
Credit facility borrowing capacity 300,000,000   $ 300,000,000
Outstanding Principal $ 155,743,000 [1] 104,005,000 [2]  
Secured Credit Facility      
Debt Instrument [Line Items]      
Credit facility borrowing capacity   300,000,000  
Outstanding Principal   $ 104,005,000  
[1] The unsecured credit facility total capacity is $300,000, of which $155,743 was outstanding as of December 31, 2018.
[2] The secured credit facility total capacity was $300,000, of which $104,005 was outstanding as of December 31, 2017.