Fair Value of Financial Instruments Fair Value Options Quantitative Disclosure (Details) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Low [Member]
Commodity Contracts
Electricity physical forwards	$ 12.24	$ 14.60
Natural gas financial swaps	1.04	0.95
High [Member]
Commodity Contracts
Electricity physical forwards	75.70	69.00
Natural gas financial swaps	4.10	4.64
Weighted Average [Member]
Commodity Contracts
Electricity physical forwards	50.85	45.00
Natural gas financial swaps	1.80	1.82
Assets [Member]
Commodity Contracts
Electricity physical forwards	4,000,000	3,000,000
Natural gas financial swaps	0	0
Total commodity contracts	4,000,000	3,000,000
Liabilities [Member]
Commodity Contracts
Electricity physical forwards	69,000,000	84,000,000
Natural gas financial swaps	5,000,000	7,000,000
Total commodity contracts	$ 74,000,000	$ 91,000,000