Accounting for Derivative Instruments and Hedging Activities Derivative Assets and Liabilities (Details) $ in Thousands, MMBTU in Millions	Jun. 30, 2020 USD ($) MMBTU	Dec. 31, 2019 USD ($) MMBTU
Derivative [Line Items]
Current, Assets	$ 27,419	$ 23,626
Long-term, Assets	7,124	7,682
Current, Liabilities	29,769	13,428
Long-term, Liabilities	34,172	12,693
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Current, Assets	27,419	23,626
Long-term, Assets	7,124	7,682
Assets	34,543	31,308
Current, Liabilities	29,769	13,428
Long-term, Liabilities	34,172	12,693
Derivative Liability	$ 63,941	$ 26,121
Not Designated as Hedging Instrument [Member] | Natural Gas Derivatives [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU	280.7	315.5
Not Designated as Hedging Instrument [Member] | Electric Portfolio [Member]
Derivative [Line Items]
Assets	$ 20,289	$ 19,933
Derivative Liability	54,238	17,504
Not Designated as Hedging Instrument [Member] | Natural Gas Portfolio [Member]
Derivative [Line Items]
Assets	14,254	11,375
Derivative Liability	$ 9,703	$ 8,617
Not Designated as Hedging Instrument [Member] | Electric Generation Fuel [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU	234.0	229.3
Not Designated as Hedging Instrument [Member] | Purchased Electricity [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount | MMBTU	7.1	10.4
Designated as Hedging Instrument
Derivative [Line Items]
Derivative Liability		$ 26,121