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Debt - Long Term Debt (Details) - USD ($) $ in Millions	9 Months Ended
Debt - Long Term Debt (Details) - USD ($) $ in Millions	Sep. 30, 2022	Mar. 10, 2022	Feb. 15, 2022	Dec. 31, 2021	Sep. 22, 2020	Jul. 02, 2018
Debt Instrument [Line Items]
Long-term Debt	$ 1,386.4			$ 1,121.9
Derivative, Gain (Loss) on Derivative, Net	0.5
Gain (Loss) on Derivative Instruments, Net, Pretax	0.4
2.400% Senior Notes due September 30, 2030
Debt Instrument [Line Items]
Long-term Debt					$ 395.8
Senior Notes, 3.800 Percent Due February 23, 2033
Debt Instrument [Line Items]
Long-term Debt			$ 395.1
5.875% Fixed-Rate Reset Junior Subordinated Debentures due 2062
Debt Instrument [Line Items]
Long-term Debt		$ 144.7
Senior Notes \| 5.000% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Long-term Debt	0.0			276.7
Stated interest rate						5.00%
Senior Notes \| 5.000% Senior Notes due September 19, 2022 \| Infinity Property and Casualty Corporation
Debt Instrument [Line Items]
Business combination, liabilities assumed, long-term debt						$ 282.1
Senior Notes \| 4.350% Senior Notes due February 15, 2025
Debt Instrument [Line Items]
Long-term Debt	$ 449.3			449.0
Stated interest rate	4.35%
Senior Notes \| 2.400% Senior Notes due September 30, 2030
Debt Instrument [Line Items]
Long-term Debt	$ 396.5			396.2
Stated interest rate					2.40%
Senior Notes \| Senior Notes, 3.800 Percent Due February 23, 2033
Debt Instrument [Line Items]
Long-term Debt	395.3			0.0
Stated interest rate			3.80%
Senior Notes \| Senior Notes, 5.875 Percent Due March 15, 2062
Debt Instrument [Line Items]
Long-term Debt	$ 145.3			$ 0.0
Junior Debt \| Senior Notes, 5.875 Percent Due March 15, 2062
Debt Instrument [Line Items]
Stated interest rate		5.875%