Fair Value Measurements (Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	$ 203.0	$ 167.7
SPH units	2.3	3.1
Assets, Fair Value Disclosure, Excluding Netting Adjustments	205.3	170.8
Netting agreements	(170.4)	(122.3)
Assets, Collateral/Margin Received or Paid	(5.0)	(2.2)
Assets from price risk management, total	27.6	43.2
Total assets at fair value	29.9	46.3
Liabilities from price risk management	198.4	154.7
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	198.4	154.7
Netting agreements	(170.4)	(122.3)
Liabilities, Collateral/Margin Received or Paid	11.4	25.7
Liabilities from price risk management, total	39.4	6.7
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	15.0	3.7
SPH units	2.3	3.1
Assets, Fair Value Disclosure, Excluding Netting Adjustments	17.3	6.8
Liabilities from price risk management	17.9	2.8
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	17.9	2.8
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	188.0	164.0
SPH units	0.0	0.0
Assets, Fair Value Disclosure, Excluding Netting Adjustments	188.0	164.0
Liabilities from price risk management	180.5	151.9
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	180.5	151.9
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	0.0	0.0
SPH units	0.0	0.0
Assets, Fair Value Disclosure, Excluding Netting Adjustments	0.0	0.0
Liabilities from price risk management	0.0	0.0
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	$ 0.0	$ 0.0