Other Comprehensive Income (Loss) And Accumulated Other Comprehensive Income - Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021	Dec. 31, 2020
Other Comprehensive Income (Loss)
Change in Net Unrecognized Postretirement Benefit Costs	$ 59.2	$ 18.9		$ (8.8)
Other Comprehensive Income (Loss) Before Income Taxes	195.6	(143.7)		(66.3)
Other Comprehensive Income (Loss) before Income Taxes
Unrealized Holding Gains and Losses	(50.3)	325.5		60.7
Tax Effects from Postretirement Benefit Plans	(12.4)	(4.0)		1.8
Other Comprehensive Income Tax (Expense) Benefit	(41.5)	30.4		14.5
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	2,505.2	2,670.6	[1]
Accumulated Net Unrealized Investment Gain (Loss)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(2.5)	(2.2)		(3.7)	$ (2.1)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(0.3)	2.0		(1.6)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	0.0	(0.5)		0.0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.3)	1.5		(1.6)
Accumulated Defined Benefit Plans Adjustment
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	9.5	(37.2)		(52.1)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	(6.0)	15.2		(6.5)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	52.7	(0.3)		0.1
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	46.7	14.9		(6.4)
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	2.5	2.8		(1.9)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	0.0	4.7		0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.3)	0.0		0.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(0.3)	4.7		0.4
Accumulated Other Comprehensive Loss
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Stockholders' Equity Attributable to Parent	(360.8)	(514.9)		(401.6)	$ (349.8)
OCI, before Reclassifications, Net of Tax, Attributable to Parent	98.2	(102.4)		(9.5)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	55.9	(10.9)		(42.3)
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	$ 154.1	$ (113.3)		$ (51.8)

[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.