Accumulated Other Comprehensive Income (Loss) - Schedule of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended								9 Months Ended
		Sep. 30, 2024		Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023		Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024		Sep. 30, 2023
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		$ 662.0		$ 664.9	$ 652.2	$ 638.1		$ 635.6	$ 631.2	$ 652.2		$ 631.2
Other comprehensive income (loss), net of tax		2.1		3.2	(2.1)	6.0		(7.3)	(1.1)	3.2		(2.4)
Ending balance		667.0		662.0	664.9	640.7		638.1	635.6	667.0		640.7
Defined Benefit Plans:
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		10.4			11.0	(1.1)			2.8	11.0		2.8
Less: income tax expense	[1]	(0.2)				(0.3)				(0.2)		(0.7)
Net loss reclassified from AOCI to Net income		0.5				0.8				0.5		2.4
Other comprehensive income (loss), net of tax		0.5				0.8				(0.1)		(3.1)
Ending balance		10.9		10.4		(0.3)		(1.1)		10.9		(0.3)
Defined Benefit Plans: Net Actuarial Gain
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Unrealized gain (loss) on securities										1.4		(0.6)
Less: income tax (expense) benefit										(0.3)		0.1
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges										1.1		(0.5)
Reclassification from AOCI	[2]	(0.3)				(0.3)				(0.8)		(0.9)
Defined Benefit Plans: Net Prior Service (Credit) Cost
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Unrealized gain (loss) on securities										(2.2)		(6.6)
Less: income tax (expense) benefit										0.5		1.6
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges										(1.7)		(5.0)
Reclassification from AOCI	[2]	1.0				1.4				1.5		4.0
Cash Flow Hedges:
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		3.8			2.1	(4.1)			0.4	2.1		0.4
Unrealized gain (loss) on securities		0.6				2.4				2.5		(9.2)
Less: income tax (expense) benefit		(0.2)				(0.6)				(0.6)		2.1
Net unrealized gain (loss) on available for sale securities, cash flow hedges and fair value hedges		0.4				1.8				1.9		(7.1)
Reclassification from AOCI		1.5				4.5				1.6		10.2
Reclassification due to forecasted transactions probable of not occurring										0.2
Less: income tax expense		(0.3)				(1.1)				(0.4)		(2.4)
Net loss reclassified from AOCI to Net income		1.2				3.4				1.4		7.8
Other comprehensive income (loss), net of tax		1.6				5.2				3.3		0.7
Ending balance		5.4	[3]	3.8		1.1	[3]	(4.1)		5.4	[3]	1.1	[3]
Accumulated Other Comprehensive Income (Loss)
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		14.2		11.0	13.1	(5.2)		2.1	3.2	13.1		3.2
Other comprehensive income (loss), net of tax		2.1		3.2	(2.1)	6.0		(7.3)	(1.1)
Ending balance		$ 16.3		$ 14.2	$ 11.0	$ 0.8		$ (5.2)	$ 2.1	$ 16.3		$ 0.8

[1] Income tax amounts reclassified out of AOCI were included as a component of Income tax provision.

[2] Amounts amortized out of AOCI related to pension and other postretirement benefits were included within Net periodic postretirement benefit cost (see Note 3).

[3] As of September 30, 2024, we estimate a net mark-to-market gain before tax of $6.2 million in AOCI will be reclassified into Net income upon settlement within the next 12 months.