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Aggregate Consolidated Scheduled Debt Repayment (Parenthetical) (Detail) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Schedule Of Equity Method Investments [Line Items]    
Line of credit borrowings $ 181,050,000 $ 201,500,000
New Revolving Credit Facility Agreement    
Schedule Of Equity Method Investments [Line Items]    
Outstanding borrowing $ 250,000,000