Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Low [Member]
Commodity Contracts
Electricity physical forwards	$ 11.29	$ 7.79
Natural gas financial swaps	1.11	1.26
Financial futures - electricity	15.33	7.79
High [Member]
Commodity Contracts
Electricity physical forwards	50.00	41.23
Natural gas financial swaps	3.00	2.92
Financial futures - electricity	33.32	29.74
Weighted Average [Member]
Commodity Contracts
Electricity physical forwards	33.35	30.95
Natural gas financial swaps	1.62	1.90
Financial futures - electricity	24.59	21.74
Assets [Member]
Commodity Contracts
Electricity physical forwards	2,000,000	0
Natural gas financial swaps	0	0
Financial futures - electricity	0	0
Total commodity contracts	2,000,000	0
Liabilities [Member]
Commodity Contracts
Electricity physical forwards	116,000,000	130,000,000
Natural gas financial swaps	15,000,000	9,000,000
Financial futures - electricity	0	0
Total commodity contracts	$ 131,000,000	$ 139,000,000