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Indebtedness - Summary Information Concerning Indebtedness (Parenthetical) (Detail) - USD ($)
Sep. 30, 2021
Dec. 31, 2020
Debt Instrument [Line Items]    
Outstanding Principal $ 1,023,011,000 $ 949,894,000
Unsecured Credit Facility    
Debt Instrument [Line Items]    
Credit facility borrowing capacity 350,000,000 350,000,000
Outstanding Principal $ 137,503,000 [1] $ 184,802,000 [2]
[1] The unsecured credit facility total capacity is $350,000, of which $137,503 was outstanding as of September 30, 2021.
[2] The unsecured credit facility total capacity was $350,000, of which $184,802 was outstanding as of December 31, 2020.