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Schedule of Highest and Lowest Outstanding Balance Under Credit Facility (Detail) - Unsecured Credit Facility - USD ($)
$ in Thousands
Oct. 07, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Unsecured credit facility $ 6,500 $ 24,000
Maximum    
Debt Instrument [Line Items]    
Unsecured credit facility $ 47,500