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Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (28.8)	$ (22.1)	$ (27.9)	$ (28.9)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.4	0.2
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(1.3)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] \| Pension Plan [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax	(34.3)	(31.5)	(35.1)	(38.8)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.8	0.7
Other Comprehensive Income (Loss), before Reclassifications, before 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	7.4	9.4	7.8	$ 9.9
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(0.4)	(0.5)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0.0
Commodity Contract [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(1.3)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ (1.9)		$ (0.6)
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