Property and Casualty Insurance Reserves - Reserve Activity (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year	[1]	$ 2,756.9
Less Reinsurance Recoverables at Beginning of Year		99.6
Incurred Losses and LAE related to:
Incurred Losses and LAE Related to Prior Years		159.8	$ (14.6)		$ 106.7
Paid Losses and LAE related to:
Plus Reinsurance Recoverables at End of Year		86.5	99.6
Property and Casualty Insurance Reserves, Gross of Reinsurance at End of Year		2,680.5	2,756.9	[1]
Property and Liability Insurance
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross of Reinsurance at Beginning of Year		2,756.9	2,772.7		1,982.5
Less Reinsurance Recoverables at Beginning of Year		39.6	41.9		50.1
Property and Casualty Insurance Reserves, Net of Reinsurance at Beginning of Year		2,717.3	2,730.8		1,932.4
Property and Casualty Insurance Reserves Acquired, Net of Reinsurance		0.0	0.0		211.1
Incurred Losses and LAE related to:
Incurred Losses and LAE Related to Current Year		3,429.9	4,103.3		4,052.7
Incurred Losses and LAE Related to Prior Years		159.8	(14.6)		106.7
Total Incurred Losses and LAE		3,589.7	4,088.7		4,159.4
Paid Losses and LAE related to:
Paid Losses and LAE Related to Current Year		1,965.3	2,460.5		2,303.4
Paid Losses and LAE Related to Prior Years		1,689.0	1,641.7		1,268.7
Total Paid Losses and LAE		3,654.3	4,102.2		3,572.1
Property and Casualty Insurance Reserves, Net of Reinsurance at End of Year		2,652.7	2,717.3		2,730.8
Plus Reinsurance Recoverables at End of Year		27.8	39.6		41.9
Property and Casualty Insurance Reserves, Gross of Reinsurance at End of Year		$ 2,680.5	$ 2,756.9		$ 2,772.7

[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.