Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Low [Member]
Commodity Contracts
Electricity physical forwards	$ 12.34	$ 7.79
Natural gas financial swaps	1.02	1.26
Financial futures - electricity	14.90	7.79
High [Member]
Commodity Contracts
Electricity physical forwards	49.11	41.23
Natural gas financial swaps	3.21	2.92
Financial futures - electricity	30.75	29.74
Weighted Average [Member]
Commodity Contracts
Electricity physical forwards	33.83	30.95
Natural gas financial swaps	1.68	1.90
Financial futures - electricity	25.15	21.74
Assets [Member]
Commodity Contracts
Electricity physical forwards	0	0
Natural gas financial swaps	0	0
Financial futures - electricity	1,000,000	0
Total commodity contracts	1,000,000	0
Liabilities [Member]
Commodity Contracts
Electricity physical forwards	112,000,000	130,000,000
Natural gas financial swaps	14,000,000	9,000,000
Financial futures - electricity	0	0
Total commodity contracts	$ 126,000,000	$ 139,000,000