Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Low [Member]
Commodity Contracts
Electricity physical forwards	$ 6.56	$ 7.79
Natural gas financial swaps	1.12	1.26
Financial futures - electricity	6.56	7.79
High [Member]
Commodity Contracts
Electricity physical forwards	44.31	41.23
Natural gas financial swaps	2.58	2.92
Financial futures - electricity	27.74	29.74
Weighted Average [Member]
Commodity Contracts
Electricity physical forwards	31.44	30.95
Natural gas financial swaps	1.59	1.90
Financial futures - electricity	18.46	21.74
Assets [Member]
Commodity Contracts
Electricity physical forwards	0	0
Natural gas financial swaps	0	0
Financial futures - electricity	0	0
Total commodity contracts	0	0
Liabilities [Member]
Commodity Contracts
Electricity physical forwards	122,000,000	130,000,000
Natural gas financial swaps	11,000,000	9,000,000
Financial futures - electricity	1,000,000	0
Total commodity contracts	$ 134,000,000	$ 139,000,000