Fair Value Measurements (Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	$ 128.0	$ 85.0
SPH units	3.7	4.3
Assets, Fair Value Disclosure, Excluding Netting Adjustments	131.7	89.3
Netting agreements	(94.2)	(67.8)
Derivative Asset, Fair Value of Collateral	(26.0)	(10.9)
Assets from price risk management, total	7.8	6.3
Total assets at fair value	11.5	10.6
Liabilities from price risk management	141.9	92.9
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	141.9	92.9
Netting agreements	(94.2)	(67.8)
Derivative Liability, Fair Value of Collateral	4.9	3.5
Liabilities from price risk management, total	52.6	28.6
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	0.9	0.6
SPH units	3.7	4.3
Assets, Fair Value Disclosure, Excluding Netting Adjustments	4.6	4.9
Liabilities from price risk management	1.6	2.7
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	1.6	2.7
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	127.1	84.4
SPH units	0.0	0.0
Assets, Fair Value Disclosure, Excluding Netting Adjustments	127.1	84.4
Liabilities from price risk management	140.3	90.2
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	140.3	90.2
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