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Components of Working Capital - Accrued Liabilities Table (Details) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Accrued Liabilities [Line Items]    
Accrued interest expense $ 34,578 $ 26,194
Short-term asset retirement obligations 2,909 3,677
Short-term remediation and reclamation obligations 1,136 1,136
Income taxes payable 931 770
Total accrued liabilities 128,380 119,096 [1]
Accrued Capital Expenditures [Member]    
Accrued Liabilities [Line Items]    
Other accrued liabilities 68,152 61,454
Accrued Plant Purchases [Member]    
Accrued Liabilities [Line Items]    
Other accrued liabilities 15,885 16,425
Other Accrued Liabilities [Member]    
Accrued Liabilities [Line Items]    
Other accrued liabilities $ 4,789 $ 9,440
[1] Financial information has been recast to include the financial position and results attributable to the Springfield system. See Note 1 and Note 2.