Fair Value of FInancial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	$ 14.25	$ 8.50
Natural gas financial swaps	1.85	2.06
Fnancial swaps - electricity	8.57	9.98
Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	54.73	84.47
Natural gas financial swaps	4.92	3.70
Fnancial swaps - electricity	33.60	27.36
Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	38.18	30.69
Natural gas financial swaps	2.64	2.54
Fnancial swaps - electricity	25.10	19.26
Assets [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	0	0
Natural gas financial swaps	1,000,000	0
Fnancial swaps - electricity	1,000,000	0
Total commodity contracts	2,000,000	0
Liabilities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	112,000,000	105,000,000
Natural gas financial swaps	9,000,000	14,000,000
Fnancial swaps - electricity	0	0
Total commodity contracts	$ 121,000,000	$ 119,000,000