Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	$ (8.0)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	746.3
Other comprehensive loss, net of tax	(8.3)
Ending balance	753.6
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(38.5)	$ (37.1)
Reclassification from AOCI, Current Period, Tax	(0.3)	(0.5)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	1.2	0.9
Other comprehensive loss, net of tax	1.2	0.9
Ending balance	(37.3)	(36.2)
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	0.2	0.2
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	1.3	1.2
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.9	0.8
OCI, before Reclassifications, before Tax, Attributable to Parent	1.0	1.1
Other Comprehensive Income (Loss) before Reclassifications, Tax	(0.2)	(0.4)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	0.8	0.7
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	(1.5)	(0.9)
Reclassification from AOCI, Current Period, Tax	0.5	0.3
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(1.0)	(0.6)
Other comprehensive loss, net of tax	(0.2)	0.1
Ending balance	0.7	0.9
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.5	(0.2)
OCI, before Reclassifications, before Tax, Attributable to Parent	(12.2)	0.3
Other Comprehensive Income (Loss) before Reclassifications, Tax	3.0	(0.1)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(9.2)	0.2
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	(0.1)	0.2
Reclassification from AOCI, Current Period, Tax	0.0	(0.1)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.1)	0.1
Other comprehensive loss, net of tax	(9.3)	0.3
Ending balance	(8.8)	0.1
Accumulated Other Comprehensive Loss (Other) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.0	(0.2)
Other comprehensive loss, net of tax	0.0	0.0
Ending balance	0.0	(0.2)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(36.7)
Other comprehensive loss, net of tax	(8.3)
Ending balance	$ (45.4)	$ (35.4)