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Commitments Contingencies and Other Matters - Additional Information (Detail) - USD ($)
 3 Months Ended 6 Months Ended
Mar. 31, 2015
Jun. 30, 2015
Commitments and Contingencies Disclosure [Line Items]    
Letters of credit, collateral for retrospective premiums and retained losses   $ 41,300,000
Amount drawn under letters of credit   0
Commitments to purchase major equipment   189,000,000
Current obligation   57,900,000
Obligation for remainder of the year   4,400,000
Settlement of fund for eligible participants $ 12,300,000  
Advances to Vendor    
Commitments and Contingencies Disclosure [Line Items]    
Advance to non-affiliates   $ 12,000,000
Notes receivable interest rate   5.00%
Notes receivable   $ 11,800,000
Repayment of notes receivable   10,400,000
Notes receivable outstanding   $ 1,400,000