Debt - Narrative (Details) - USD ($)			1 Months Ended	12 Months Ended
	Feb. 23, 2022	Jun. 04, 2019	Jun. 30, 2017	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021		Mar. 10, 2022	Feb. 15, 2022	Sep. 22, 2020	Jul. 02, 2018	Jun. 08, 2018	Feb. 28, 2015
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2023 - $1,213.4; 2022 - $1,195.1)				$ 1,389,200,000	$ 1,386,900,000	[1]
Loss from Early Extinguishment of Debt				0	3,700,000	[2]	$ 0	[3]
Interest expense				56,100,000	54,700,000		43,600,000
Interest paid including facility fees				54,500,000	51,500,000		43,900,000
Net gain on derivative instrument				500,000
Gain on derivative				$ 5,900,000
Derivative, Gain, Statement of Income or Comprehensive Income [Extensible Enumeration]				(Loss) Gain on Cash Flow Hedges
Amortization				$ 600,000
Federal Home Loan Bank, amount of advances				0	81,000,000		$ 85,000,000
Federal Home Loan Bank, amount of advances, Outstanding				0	0
Senior Notes, 3.800 Percent Due February 23, 2032
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2023 - $1,213.4; 2022 - $1,195.1)										$ 395,100,000
5.875% Fixed-Rate Reset Junior Subordinated Debentures due 2062
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2023 - $1,213.4; 2022 - $1,195.1)									$ 144,700,000
Senior Notes | 5.000% Senior Notes Due September 19, 2022
Debt Instrument [Line Items]
Redemption price, percentage	100.00%
Senior Notes | Senior Notes, 4.35 Percent Due February 15, 2025
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2023 - $1,213.4; 2022 - $1,195.1)				$ 449,600,000	449,300,000
Stated interest rate, percentage				4.35%
Debt instrument, face amount														$ 250,000,000
Increase in debt instrument			$ 200,000,000
Long-term debt, gross				$ 450,000,000
Senior Notes | Senior Notes, 2.400 Percent Due September 30, 2030
Debt Instrument [Line Items]
Proceeds from debt, net of issuance costs				395,800,000
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2023 - $1,213.4; 2022 - $1,195.1)				397,000,000.0	396,600,000
Stated interest rate, percentage											2.40%
Debt instrument, effective interest rate											2.52%
Long-term debt, gross				400,000,000
Senior Notes | Senior Notes, 3.800 Percent Due February 23, 2032
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2023 - $1,213.4; 2022 - $1,195.1)				396,000,000.0	395,500,000
Stated interest rate, percentage										3.80%
Debt instrument, effective interest rate											3.95%
Long-term debt, gross				400,000,000
Senior Notes | 5.875% Fixed-Rate Reset Junior Subordinated Debentures due 2062
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2023 - $1,213.4; 2022 - $1,195.1)				146,600,000	145,500,000
Long-term debt, gross				150,000,000
Revolving Credit Facility | Notes Payable under Revolving Credit Agreement | Second Amended and Restated Credit Agreement
Debt Instrument [Line Items]
Maximum Capacity Amount		$ 800,000,000		$ 393,000,000									$ 600,000,000
Increase in line of credit facility		200,000,000
Debt issuance costs		2,200,000
Unamortized debt issuance expense		$ 1,700,000
Line of credit, outstanding					$ 0
Infinity Property and Casualty Corporation | Senior Notes | 5.000% Senior Notes Due September 19, 2022
Debt Instrument [Line Items]
Debt instrument, face amount												$ 275,000,000

[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.

[2] 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.

[3] 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.