Fair Value of FInancial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Minimum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	$ 14.60	$ 7.79
Natural gas financial swaps	0.95	1.26
Fnancial swaps - electricity	20.75	7.79
Maximum [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	69.00	41.23
Natural gas financial swaps	4.64	2.92
Fnancial swaps - electricity	35.46	29.74
Weighted Average [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	45.00	30.95
Natural gas financial swaps	1.82	1.90
Fnancial swaps - electricity	28.63	21.74
Assets [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	3,000,000	0
Natural gas financial swaps	0	0
Fnancial swaps - electricity	0	0
Total commodity contracts	3,000,000	0
Liabilities [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Electricity physical forward purchase	84,000,000	130,000,000
Natural gas financial swaps	7,000,000	9,000,000
Fnancial swaps - electricity	0	0
Total commodity contracts	$ 91,000,000	$ 139,000,000