Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) (USD $)	Sep. 30, 2012	Dec. 31, 2011
Minimum [Member]
Assets from price risk management activities: [Abstract]
Natural gas financial swaps	$ 3.70
Liabilities from price risk management activities:
Electricity financial swaps	6.05
Natural gas financial swaps	3.29
Maximum [Member]
Assets from price risk management activities: [Abstract]
Natural gas financial swaps	5.41
Liabilities from price risk management activities:
Electricity financial swaps	51.68
Natural gas financial swaps	4.94
Weighted Average [Member]
Assets from price risk management activities: [Abstract]
Natural gas financial swaps	4.33
Liabilities from price risk management activities:
Electricity financial swaps	39.44
Natural gas financial swaps	4.03
Fair Value, Inputs, Level 3 [Member]
Assets from price risk management activities: [Abstract]
Natural gas financial swaps	4,000,000	0
Liabilities from price risk management activities:
Electricity financial swaps	33,000,000	29,000,000
Natural gas financial swaps	$ 51,000,000	$ 50,000,000