Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (22.1)	$ (23.0)	$ (28.9)	$ (23.2)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.2	0.2
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax	(31.5)	(32.0)	(38.8)	(32.7)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.7	0.7
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	4.6	2.5	4.6	2.5
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	0.0
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	0.0	0.0	0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0	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	4.8	6.5	$ 5.3	$ 7.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.5)	$ (0.5)
Settlement Cost [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	6.6
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	6.6
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
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