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Indebtedness - Summary Information Concerning Indebtedness (Parenthetical) (Detail) - USD ($)
Dec. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]    
Outstanding Principal $ 949,894,000 $ 991,178,000
Unsecured Credit Facility    
Debt Instrument [Line Items]    
Credit facility borrowing capacity 350,000,000  
Outstanding Principal $ 184,802,000 [1] 186,302,000 [2]
Secured Credit Facility    
Debt Instrument [Line Items]    
Credit facility borrowing capacity   350,000,000
Outstanding Principal   $ 186,302,000
[1] The unsecured credit facility total capacity is $350,000, of which $184,802 was outstanding as of December 31, 2020.
[2] The secured credit facility total capacity was $350,000, of which $186,302 was outstanding as of December 31, 2019.