Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Unrealized Gain (Loss) [Abstract]
Accumulated Other Comprehensive Income	$ (360.8)	$ (514.9)	[1]
AOCI, Gain (Loss), Debt Securities, Available-for-sale, with Allowance for Credit Loss, Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(50.3)	325.9		$ 59.7
Reclassification from AOCI, Current Period, Tax	(0.9)	2.7		11.3
Accumulated Net Unrealized Investment Gain (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	0.2	(0.4)		0.4
Reclassification from AOCI, Current Period, Tax	0.0	0.1		0.0
Accumulated Defined Benefit Plans Adjustment
Debt Securities, Unrealized Gain (Loss) [Abstract]
Net Unrecognized Postretirement Benefit Costs, Net of Income Taxes					$ (45.7)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(12.5)	(4.0)		2.4
Reclassification from AOCI, Current Period, Tax	(13.8)	0.1		0.0
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(0.1)	(1.2)		(0.1)
Reclassification from AOCI, Current Period, Tax	(0.1)	0.0		(0.1)
Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	(41.5)	30.4		14.5
Reclassification from AOCI, Current Period, Tax	$ (14.8)	$ 2.9		$ 11.2

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.