Acquired Sales Contracts (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2014
Supply Commitment [Line Items]
Impairment of Intangible Assets, Finite-lived	$ 15,700,000
Acquired fair value, Assets	131,819,000	131,299,000
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months		12,200,000
Finite-Lived Intangible Assets, Amortization Expense, Year Two		7,200,000
Finite-Lived Intangible Assets, Amortization Expense, Year Three		3,600,000
Finite-Lived Intangible Assets, Amortization Expense, Year Four		3,600,000
Finite-Lived Intangible Assets, Amortization Expense, Year Five		4,100,000
imparment of intangible asset [Member]
Supply Commitment [Line Items]
Acquired fair value, Assets	$ 17,500,000