Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Low [Member]
Commodity Contracts
Electricity physical forwards	$ 8.20	$ 14.25
Natural gas financial swaps	1.59	1.85
Financial futures - electricity	8.20	8.57
High [Member]
Commodity Contracts
Electricity physical forwards	37.15	54.73
Natural gas financial swaps	3.22	4.92
Financial futures - electricity	29.50	33.60
Weighted Average [Member]
Commodity Contracts
Electricity physical forwards	28.36	38.18
Natural gas financial swaps	2.07	2.64
Financial futures - electricity	23.05	25.10
Assets [Member]
Commodity Contracts
Electricity physical forwards	0	0
Natural gas financial swaps	0	1,000,000
Financial futures - electricity	0	1,000,000
Total commodity contracts	0	2,000,000
Liabilities [Member]
Commodity Contracts
Electricity physical forwards	140,000,000	112,000,000
Natural gas financial swaps	13,000,000	9,000,000
Financial futures - electricity	0	0
Total commodity contracts	$ 153,000,000	$ 121,000,000