Fair Value Measurements (Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	$ 263.1	$ 537.1
Other investments	2.8	2.6
Total assets at fair value	265.9	539.7
Netting agreements	(213.9)	(452.1)
Assets, Collateral/Margin Received or Paid	(11.2)	(12.2)
Assets from price risk management, total	38.0	72.8
Total assets at fair value	40.8	75.4
Collateral Already Posted, Aggregate Fair Value	3.9	25.6
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	223.2	497.7
Netting agreements	(213.9)	(452.1)
Liabilities, Collateral/Margin Received or Paid	8.3	21.7
Gross Liabilities From Price Risk Management With Credit-Risk-Related Contingent Features	217.7	485.8
Aggregate fair value of commodity derivative instruments	7.7	8.1
Liabilities from price risk management without credit-risk-related contingent features	5.5	11.9
Additional Collateral, Aggregate Fair Value	4.4	3.9
Derivative, Net Liability Position, Aggregate Fair Value, Without Credit-Risk Contingent Features	9.9	15.8
Liabilities, Fair Value Disclosure, Total	17.6	23.9
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	14.5	62.8
Other investments	2.8	2.6
Total assets at fair value	17.3	65.4
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	11.5	65.7
Gross Liabilities From Price Risk Management With Credit-Risk-Related Contingent Features	11.5	65.7
Liabilities from price risk management without credit-risk-related contingent features	0.0	0.0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	248.6	474.3
Other investments	0.0	0.0
Total assets at fair value	248.6	474.3
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	211.7	432.0
Gross Liabilities From Price Risk Management With Credit-Risk-Related Contingent Features	206.2	420.1
Liabilities from price risk management without credit-risk-related contingent features	5.5	11.9
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	0.0	0.0
Other investments	0.0	0.0
Total assets at fair value	0.0	0.0
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	0.0	0.0
Gross Liabilities From Price Risk Management With Credit-Risk-Related Contingent Features	0.0	0.0
Liabilities from price risk management without credit-risk-related contingent features	$ 0.0	$ 0.0