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Concentrations of Credit Risk - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Risks And Uncertainties [Abstract]      
Provision for bad debts $ 0us-gaap_ProvisionForDoubtfulAccounts $ 0us-gaap_ProvisionForDoubtfulAccounts $ 1,100us-gaap_ProvisionForDoubtfulAccounts