Fair Value Measurements (Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	$ 214.0	$ 167.7
SPH units	2.0	3.1
Assets, Fair Value Disclosure, Excluding Netting Adjustments	216.0	170.8
Netting agreements	(186.3)	(122.3)
Assets, Collateral/Margin Received or Paid	16.7	2.2
Assets from price risk management, total	44.4	43.2
Total assets at fair value	46.4	46.3
Liabilities from price risk management	207.5	154.7
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	207.5	154.7
Netting agreements	(186.3)	(122.3)
Liabilities, Collateral/Margin Received or Paid	11.0	25.7
Liabilities from price risk management, total	32.2	6.7
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	26.2	3.7
SPH units	2.0	3.1
Assets, Fair Value Disclosure, Excluding Netting Adjustments	28.2	6.8
Liabilities from price risk management	29.7	2.8
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	29.7	2.8
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	187.8	164.0
SPH units	0.0	0.0
Assets, Fair Value Disclosure, Excluding Netting Adjustments	187.8	164.0
Liabilities from price risk management	177.8	151.9
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	177.8	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