Fair Value Measurements (Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - Fair Value, Measurements, Recurring - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	$ 42.4	$ 85.0
SPH units	3.4	4.3
Assets, Fair Value Disclosure, Excluding Netting Adjustments	45.8	89.3
Netting agreements	(30.2)	(67.8)
Derivative Asset, Fair Value of Collateral	0.6	(10.9)
Assets from price risk management, total	12.8	6.3
Total assets at fair value	16.2	10.6
Liabilities from price risk management	36.9	92.9
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	36.9	92.9
Netting agreements	(30.2)	(67.8)
Derivative Liability, Fair Value of Collateral	1.3	3.5
Liabilities from price risk management, total	8.0	28.6
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	0.7	0.6
SPH units	3.4	4.3
Assets, Fair Value Disclosure, Excluding Netting Adjustments	4.1	4.9
Liabilities from price risk management	0.3	2.7
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	0.3	2.7
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets from price risk management	41.7	84.4
SPH units	0.0	0.0
Assets, Fair Value Disclosure, Excluding Netting Adjustments	41.7	84.4
Liabilities from price risk management	36.6	90.2
Liabilities, Fair Value Disclosure, Excluding Netting Adjustments	36.6	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