Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Low [Member]
Commodity Contracts
Natural gas financial swaps	$ 3.58	$ 3.67
Electricity financial swaps	7.32	7.12
Electricity physical forward purchase	41.22
High [Member]
Commodity Contracts
Natural gas financial swaps	5.02	5.21
Electricity financial swaps	48.59	51.72
Electricity physical forward purchase	43.74
Weighted Average [Member]
Commodity Contracts
Natural gas financial swaps	4.20	4.28
Electricity financial swaps	39.65	41.14
Electricity physical forward purchase	42.60
Assets [Member]
Commodity Contracts
Natural gas financial swaps	1,000,000	2,000,000
Electricity financial swaps	0	0
Electricity physical forward purchase	0
Total commodity contracts	1,000,000	2,000,000
Liabilities [Member]
Commodity Contracts
Natural gas financial swaps	10,000,000	8,000,000
Electricity financial swaps	12,000,000	10,000,000
Electricity physical forward purchase	24,000,000
Total commodity contracts	$ 46,000,000	$ 18,000,000