Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($)	Jun. 30, 2017	Dec. 31, 2016
Low [Member]
Commodity Contracts
Electricity physical forward	$ 12.25	$ 14.25
Natural gas financial swaps	1.70	1.85
Fnancial swaps - electricity	15.83	8.57
High [Member]
Commodity Contracts
Electricity physical forward	35.56	54.73
Natural gas financial swaps	3.15	4.92
Fnancial swaps - electricity	29.94	33.60
Weighted Average [Member]
Commodity Contracts
Electricity physical forward	27.90	38.18
Natural gas financial swaps	2.14	2.64
Fnancial swaps - electricity	24.37	25.10
Assets [Member]
Commodity Contracts
Electricity physical forward	0	0
Natural gas financial swaps	0	1,000,000
Fnancial swaps - electricity	2,000,000	1,000,000
Total commodity contracts	2,000,000	2,000,000
Liabilities [Member]
Commodity Contracts
Electricity physical forward	143,000,000	112,000,000
Natural gas financial swaps	11,000,000	9,000,000
Fnancial swaps - electricity	1,000,000	0
Total commodity contracts	$ 155,000,000	$ 121,000,000