Components of Working Capital - Accrued Liabilities Table (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Accrued Liabilities [Line Items]
Accrued interest expense	$ 39,834	$ 26,194
Short-term asset retirement obligations	3,114	3,677
Short-term remediation and reclamation obligations	630	1,136
Income taxes payable	1,006	770
Total accrued liabilities	168,981	119,096
Accrued Capital Expenditures [Member]
Accrued Liabilities [Line Items]
Other accrued liabilities	79,253	61,454
Accrued Plant Purchases [Member]
Accrued Liabilities [Line Items]
Other accrued liabilities	44,538	16,425
Other Accrued Liabilities [Member]
Accrued Liabilities [Line Items]
Other accrued liabilities	$ 606	$ 9,440