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Debt and Credit Facilities - Summary of Debt (Detail) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Total principal outstanding $ 590,474 $ 684,296
Current maturities (166,684) (254,398)
Debt issuance costs (1,025) (1,091)
Long-term debt 422,765 428,807
Unsecured Senior Notes [Member]    
Debt Instrument [Line Items]    
Total principal outstanding 500,000 500,000
Equipment Financing Notes [Member]    
Debt Instrument [Line Items]    
Total principal outstanding 40,474 49,296
Trade Receivables [Member] | Collateralized [Member] | Secured Credit Facility [Member]    
Debt Instrument [Line Items]    
Total principal outstanding $ 50,000 $ 135,000