Fair Value of FInancial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($)	Dec. 31, 2015	Dec. 31, 2014
Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	$ 8.50	$ 11.97
Natural gas financial swaps	2.06	2.88
Fnancial swaps - electricity	9.98	11.97
Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	84.47	122.72
Natural gas financial swaps	3.70	4.86
Fnancial swaps - electricity	27.36	39.26
Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	30.69	37.43
Natural gas financial swaps	2.54	3.41
Fnancial swaps - electricity	19.26	27.88
Assets [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	0	0
Natural gas financial swaps	0	0
Fnancial swaps - electricity	0	1,000,000
Total commodity contracts	0	1,000,000
Liabilities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	105,000,000	77,000,000
Natural gas financial swaps	14,000,000	21,000,000
Fnancial swaps - electricity	0	3,000,000
Total commodity contracts	$ 119,000,000	$ 101,000,000