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Summary Information Concerning Indebtedness (Parenthetical) (Detail) - USD ($)
Mar. 31, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Outstanding Principal $ 909,404,000 $ 784,640,000
Unsecured Credit Facility    
Debt Instrument [Line Items]    
Secured credit facility borrowing capacity 300,000,000  
Outstanding Principal $ 190,005,000 [1] 104,005,000 [2]
Secured Credit Facility    
Debt Instrument [Line Items]    
Secured credit facility borrowing capacity   300,000,000
Outstanding Principal   $ 104,005,000
[1] The unsecured credit facility total capacity is $300,000, of which $190,005 was outstanding as of March 31, 2018.
[2] The secured credit facility total capacity was $300,000, of which $104,005 was outstanding as of December 31, 2017.