Components of Working Capital - Accrued Liabilities Table (details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Accrued Liabilities [Line Items]
Other accrued liabilities	$ 1,181		$ 1,241
Accrued interest expense	24,741		18,119
Short-term asset retirement obligations	1,212		1,966
Short-term remediation and reclamation obligations	475		562
Income taxes payable	207		0
Total accrued liabilities	158,730	[1]	138,034	[1]
Accrued Capital Expenditures [Member]
Accrued Liabilities [Line Items]
Other accrued liabilities	116,891		94,750
Accrued Plant Purchases [Member]
Accrued Liabilities [Line Items]
Other accrued liabilities	$ 14,023		$ 21,396

[1] Accrued liabilities includes amounts payable to affiliates of zero and $0.1 million as of December 31, 2014 and 2013, respectively.