Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 733.9	$ 740.4
Other comprehensive income (loss), net of tax	(15.5)	9.7
Ending balance	721.1	747.9
Estimated net mark-to-market loss before tax within next twelve months - cash flow hedges	18.1
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(33.2)	(35.6)
Less: income tax (expense) benefit	(0.3)	(0.4)
Net loss (gain) reclassified from AOCI to Net income	1.0	1.1
Other comprehensive income (loss), net of tax	1.1	1.1
Ending balance	(32.1)	(34.5)
Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification of unrealized gain upon sale of available for sale securities and Excluded component amortized from AOCI to net income	0.1	0.1
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification of unrealized gain upon sale of available for sale securities and Excluded component amortized from AOCI to net income	1.2	1.4
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.3	0.3
Unrealized gain (loss) on securities	1.3	1.0
Less: income tax benefit (expense)	(0.3)	(0.3)
Net unrealized (loss) gain on cash flow hedges	1.0	0.7
Reclassification of unrealized gain upon sale of available for sale securities and Excluded component amortized from AOCI to net income	(1.5)	(1.2)
Less: income tax (expense) benefit	0.3	0.3
Net loss (gain) reclassified from AOCI to Net income	(1.2)	(0.9)
Other comprehensive income (loss), net of tax	(0.2)	(0.2)
Ending balance	0.1	0.1
AOCI, Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(5.6)	(13.4)
Unrealized gain (loss) on securities	(27.9)	7.4
Less: income tax benefit (expense)	6.8	(1.8)
Net unrealized (loss) gain on cash flow hedges	(21.1)	5.6
Reclassification of unrealized gain upon sale of available for sale securities and Excluded component amortized from AOCI to net income	7.0	4.2
Less: income tax (expense) benefit	(1.8)	(1.0)
Net loss (gain) reclassified from AOCI to Net income	5.2	3.2
Other comprehensive income (loss), net of tax	(15.9)	8.8
Ending balance	(21.5)	(4.6)
AOCI, Excluded Component of Derivative Qualifying as Fair Value Hedge Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.0	0.0
Unrealized gain (loss) on securities	(0.8)	0.0
Less: income tax benefit (expense)	0.2	0.0
Net unrealized (loss) gain on cash flow hedges	(0.6)	0.0
Reclassification of unrealized gain upon sale of available for sale securities and Excluded component amortized from AOCI to net income	0.2	0.0
Less: income tax (expense) benefit	(0.1)	0.0
Net loss (gain) reclassified from AOCI to Net income	0.1	0.0
Other comprehensive income (loss), net of tax	(0.5)	0.0
Ending balance	(0.5)	0.0
AOCI, Accumulated Foreign Currency Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.1)	(0.1)
Other comprehensive income (loss), net of tax	0.0	0.0
Ending balance	(0.1)	(0.1)
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(38.6)	(48.8)
Other comprehensive income (loss), net of tax	(15.5)	9.7
Ending balance	(54.1)	$ (39.1)
Fair Value Hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Estimated net mark-to-market loss before tax within next twelve months - fair value hedges	$ 0.6