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Borrowing Limits Amounts Outstanding and Amounts Available under Revolving Credit Facility (Detail) - Revolving Credit Facility - USD ($)
Jun. 30, 2015
Dec. 31, 2014
Jun. 30, 2014
Oct. 04, 2013
Debt Instrument [Line Items]        
Revolving credit facility $ 120,000,000 $ 120,000,000 $ 120,000,000 $ 120,000,000
Outstanding revolving credit facility   29,300,000 35,000,000  
Available revolving credit facility $ 120,000,000 $ 90,700,000 $ 85,000,000