Discontinued Operations and Disposal Groups (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Available-for-sale Securities		$ 12,139.9	$ 11,746.7
Short-term investments		1,032.6	860.8
Total investments		15,867.7	15,859.9
Cash		202.8	81.0
Accrued investment income		117.0	106.7
Accounts and notes receivable		2,201.4	1,927.5
Federal income tax recoverable: Current		21.8	15.7
Total assets		26,501.4	25,159.4
Loss and loss adjustment expense reserves		12,538.2	12,221.5	$ 11,425.5	$ 10,671.0
Unearned premiums		3,042.7	2,787.8
Total policy liabilities		15,731.4	15,191.6
Commissions, expenses, fees, and taxes		533.8	514.8
Other liabilities		690.6	560.5
Total liabilities		20,090.7	$ 18,986.2
Disposal Group, Including Discontinued Operation, Consideration		140.0
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal		45.6
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down		$ 34.5
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal, Statement of Income or Comprehensive Income [Extensible Enumeration]		Consolidated revenues
Transactioncosts
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal		$ 11.0
Disposal Group, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Available-for-sale Securities		29.8
Short-term investments		191.3
Total investments		221.2
Cash		0.8
Accrued investment income		0.9
Accounts and notes receivable		0.5
Federal income tax recoverable: Current		2.2
Deferred Income Tax Assets, Net		0.2
Other assets	[1]	(31.1)
Total assets		194.8
Loss and loss adjustment expense reserves		54.9
Unearned premiums		0.1
Total policy liabilities		55.0
Commissions, expenses, fees, and taxes		1.2
Other liabilities		0.5
Total liabilities		$ 56.8

[1] Other assets is presented net of a valuation allowance of $34.5 recorded upon remeasurement of the disposal group to fair value.