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Summary Information Concerning Indebtedness (Parenthetical) (Detail) - USD ($)
Sep. 30, 2019
Dec. 31, 2018
Debt Instrument [Line Items]    
Outstanding Principal $ 985,016,000 $ 991,415,000
Unsecured Credit Facility    
Debt Instrument [Line Items]    
Credit facility borrowing capacity 350,000,000 300,000,000
Outstanding Principal $ 156,303,000 [1] $ 155,743,000 [2]
[1] The total capacity under the unsecured credit facility is $350,000, of which $156,303 was outstanding as of September 30, 2019.
[2] The total capacity under the unsecured credit facility was $300,000, of which $155,743 was outstanding as of December 31, 2018.