Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (18.9)	$ (20.3)	$ (23.2)	$ (29.3)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1.2	1.8
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		9.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		6.7
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax	(27.1)	(30.1)	(32.7)	(32.1)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	2.5	2.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		2.0
Other Comprehensive Income (Loss), before Reclassifications, before Tax		0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		0.0
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax	2.5	3.2	2.5	2.7
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		0.5
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Pension Plan [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax	0.0	0.1	0.0	0.1
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		0.0
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Other Postretirement Benefits Plan [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax	5.7	6.5	$ 7.0	$ 0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(1.3)	(0.2)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		6.5
Defined Benefit Plans Income Loss [Member] | Other Postretirement Benefits Plan [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		0.0
Defined Benefit Plan Prior Service Cost [Member] | Other Postretirement Benefits Plan [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		6.7
Settlement Cost [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	3.1	0.5
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	3.1	0.0
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	0.5
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Pension Plan [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0.0
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Other Postretirement Benefits Plan [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ 0.0	$ 0.0