Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Estimated net mark-to-market loss before tax within next twelve months	$ (2.5)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			$ 746.3
Other comprehensive (loss) income, net of tax			(1.9)
Ending balance	761.0		761.0
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(37.3)	$ (36.2)	(38.5)	$ (37.1)
Less: income tax benefit	(0.4)	(0.6)	(0.7)	(1.1)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	1.2	0.8	2.4	1.7
Other comprehensive (loss) income, net of tax	1.2	0.8	2.4	1.7
Ending balance	(36.1)	(35.4)	(36.1)	(35.4)
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification of unrealized gain upon sale of available for sale securities	0.2	0.2	0.4	0.4
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification of unrealized gain upon sale of available for sale securities	1.4	1.2	2.7	2.4
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.7	0.9	0.9	0.8
Unrealized gain on available for sale securities	1.3	1.0	2.3	2.1
Less: income tax expense	(0.4)	(0.4)	(0.6)	(0.8)
Net unrealized gain on available for sale securities	0.9	0.6	1.7	1.3
Reclassification of unrealized gain upon sale of available for sale securities	(1.2)	(0.7)	(2.7)	(1.6)
Less: income tax benefit	0.2	0.3	0.7	0.6
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(1.0)	(0.4)	(2.0)	(1.0)
Other comprehensive (loss) income, net of tax	(0.1)	0.2	(0.3)	0.3
Ending balance	0.6	1.1	0.6	1.1
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(8.8)	0.1	0.5
Unrealized gain on available for sale securities	10.9	(0.1)	(1.3)	0.2
Less: income tax expense	(2.6)	0.0	0.4	(0.1)
Net unrealized gain on available for sale securities	8.3	(0.1)	(0.9)	0.1
Reclassification of unrealized gain upon sale of available for sale securities	(3.9)	0.1	(4.0)	0.3
Less: income tax benefit	0.9	0.0	0.9	(0.1)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(3.0)	0.1	(3.1)	0.2
Other comprehensive (loss) income, net of tax	5.3	0.0	(4.0)	0.3
Ending balance	(3.5)	0.1	(3.5)	0.1
Accumulated Other Comprehensive Loss (Other) [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.0	(0.2)	0.0	(0.2)
Other comprehensive (loss) income, net of tax	0.0	0.1	0.0	0.1
Ending balance	0.0	(0.1)	0.0	(0.1)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(36.7)
Other comprehensive (loss) income, net of tax			(1.9)
Ending balance	$ (39.0)	$ (34.3)	$ (39.0)	$ (34.3)