Fair Value of FInancial Instruments Fair Value Options Quantitative Disclosure (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Option, Quantitative Disclosures [Line Items]
Commodity Contract Liabilities, Electricity	$ 82,000,000	$ 137,000,000
Commodity Contract Liabilities, Natural gas	118,000,000	251,000,000
Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
commodity contracts assets, natural gas	3.74
Commodity Contract Liabilities, Electricity	7.12
Commodity Contract Liabilities, Natural gas	3.67
Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
commodity contracts assets, natural gas	5.21
Commodity Contract Liabilities, Electricity	51.72
Commodity Contract Liabilities, Natural gas	5.21
Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
commodity contracts assets, natural gas	4.36
Commodity Contract Liabilities, Electricity	41.14
Commodity Contract Liabilities, Natural gas	4.20
Fair Value, Inputs, Level 3 [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
commodity contracts assets, natural gas	2,000,000
Commodity Contract Liabilities, Electricity	10,000,000	29,000,000
Commodity Contract Liabilities, Natural gas	$ 8,000,000	$ 50,000,000