Acquired Sales Contracts Acquired Sales Contracts (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Supply Commitment [Line Items]
Impairment of Intangible Assets, Finite-lived	$ 15,700,000
Acquired fair value, Assets	131,819,000	149,457,000
Finite-Lived Intangible Assets, Amortization Expense, Next Twelve Months	8,100,000
Finite-Lived Intangible Assets, Amortization Expense, Year Two	11,600,000
Finite-Lived Intangible Assets, Amortization Expense, Year Three	10,800,000
Finite-Lived Intangible Assets, Amortization Expense, Year Four	9,300,000
Finite-Lived Intangible Assets, Amortization Expense, Year Five	3,200,000
imparment of intangible asset [Member]
Supply Commitment [Line Items]
Acquired fair value, Assets	$ 17,500,000