Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Estimated net mark-to-market loss before tax within next twelve months							$ 12.2
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 746.3	$ 747.9	$ 740.4	$ 761.0	$ 753.6	$ 746.3	740.4	$ 746.3
Other comprehensive income (loss), net of tax	(0.9)	(3.2)	9.7	(0.8)	6.4	(8.3)
Ending balance	751.4	746.3	747.9	762.5	761.0	753.6	751.4	762.5
Accumulated Defined Benefit Plans Adjustment Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(33.4)		(35.6)	(36.1)		(38.5)	(35.6)	(38.5)
Less: income tax (expense) benefit	(0.3)			(0.4)			(1.1)	(1.1)
Net loss (gain) reclassified from AOCI to Net income	1.2			1.1			3.4	3.5
Other comprehensive income (loss), net of tax	1.2			1.1			3.4	3.5
Ending balance	(32.2)	(33.4)		(35.0)	(36.1)		(32.2)	(35.0)
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	0.1			0.2			0.3	0.6
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	1.4			1.3			4.2	4.0
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	0.0		0.3	0.6		0.9	0.3	0.9
Unrealized gain (loss) on securities	1.0			1.3			2.7	3.6
Less: income tax benefit	(0.3)			(0.3)			(0.7)	(0.9)
Net unrealized loss on cash flow hedges	0.7			1.0			2.0	2.7
Reclassification of unrealized gain upon sale of available for sale securities	(0.9)			(1.6)			(3.0)	(4.3)
Less: income tax (expense) benefit	0.3			0.4			0.8	1.1
Net loss (gain) reclassified from AOCI to Net income	(0.6)			(1.2)			(2.2)	(3.2)
Other comprehensive income (loss), net of tax	0.1			(0.2)			(0.2)	(0.5)
Ending balance	0.1	0.0		0.4	0.6		0.1	0.4
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(8.8)		(13.4)	(3.5)		0.5	(13.4)	0.5
Unrealized gain (loss) on securities	(8.4)			(5.2)			(11.1)	(6.5)
Less: income tax benefit	2.1			1.2			2.7	1.6
Net unrealized loss on cash flow hedges	(6.3)			(4.0)			(8.4)	(4.9)
Reclassification of unrealized gain upon sale of available for sale securities	5.4			3.0			14.3	(1.0)
Less: income tax (expense) benefit	(1.3)			(0.7)			(3.5)	0.2
Net loss (gain) reclassified from AOCI to Net income	4.1			2.3			10.8	(0.8)
Other comprehensive income (loss), net of tax	(2.2)			(1.7)			2.4	(5.7)
Ending balance	(11.0)	(8.8)		(5.2)	(3.5)		(11.0)	(5.2)
Accumulated Other Comprehensive Loss (Other)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(0.1)		(0.1)	0.0		0.0	(0.1)	0.0
Other comprehensive income (loss), net of tax	0.0			0.0			0.0	0.0
Ending balance	(0.1)	(0.1)		0.0	0.0		(0.1)	0.0
AOCI Attributable to Parent
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(42.3)	(39.1)	(48.8)	(39.0)	(45.4)	(36.7)	(48.8)	(36.7)
Other comprehensive income (loss), net of tax	(0.9)	(3.2)	9.7	(0.8)	6.4	(8.3)
Ending balance	$ (43.2)	$ (42.3)	$ (39.1)	$ (39.8)	$ (39.0)	$ (45.4)	$ (43.2)	$ (39.8)