Debt - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended		9 Months Ended
	Jun. 04, 2019	Mar. 31, 2021	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Mar. 10, 2022	Feb. 15, 2022	Dec. 31, 2021	Sep. 22, 2020	Jul. 02, 2018	Jun. 08, 2018	Feb. 28, 2015
Debt Instrument [Line Items]
Long-term Debt			$ 1,386,400,000		$ 1,386,400,000				$ 1,121,900,000
Interest and other expenses			14,300,000	$ 10,700,000	41,000,000	$ 33,000,000
Interest paid, including capitalized interest, operating and investing activities			25,000,000	$ 21,600,000	49,400,000	$ 43,700,000
Derivative, Gain on Derivative					5,900,000
2.400% Senior Notes due September 30, 2030
Debt Instrument [Line Items]
Long-term Debt										$ 395,800,000
Senior Notes, 3.800 Percent Due February 23, 2033
Debt Instrument [Line Items]
Long-term Debt								$ 395,100,000
5.875% Fixed-Rate Reset Junior Subordinated Debentures due 2062
Debt Instrument [Line Items]
Long-term Debt							$ 144,700,000
Senior Notes | 5.000% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Long-term Debt			0		0				276,700,000
Stated interest rate											5.00%
Senior Notes | 5.000% Senior Notes due September 19, 2022 | Infinity Property and Casualty Corporation
Debt Instrument [Line Items]
Face amount of debt											$ 275,000,000
Business combination, liabilities assumed, long-term debt											282,100,000
Unamortized premium											$ 7,100,000
Effective interest rate											4.36%
Senior Notes | 4.350% Senior Notes due February 15, 2025
Debt Instrument [Line Items]
Long-term Debt			$ 449,300,000		$ 449,300,000				449,000,000.0
Stated interest rate			4.35%		4.35%
Long-term debt, gross			$ 450,000,000		$ 450,000,000
Face amount of debt													$ 250,000,000
Increase in senior notes		$ 200,000,000
Senior Notes | 2.400% Senior Notes due September 30, 2030
Debt Instrument [Line Items]
Long-term Debt			396,500,000		396,500,000				396,200,000
Stated interest rate										2.40%
Long-term debt, gross			400,000,000		400,000,000
Effective interest rate										2.52%
Senior Notes | Senior Notes, 3.800 Percent Due February 23, 2033
Debt Instrument [Line Items]
Long-term Debt			395,300,000		395,300,000				$ 0
Stated interest rate								3.80%
Long-term debt, gross			400,000,000		400,000,000
Effective interest rate										3.95%
Senior Notes | 5.875% Fixed-Rate Reset Junior Subordinated Debentures due 2062
Debt Instrument [Line Items]
Long-term debt, gross			150,000,000		150,000,000
Revolving Credit Facility | Notes Payable under Revolving Credit Agreement | Second Amended and Restated Credit Agreement
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800,000,000											$ 600,000,000
Increase in revolving credit borrowing capacity	200,000,000
Debt issuance costs	2,200,000
Remaining unamortized costs	$ 2,300,000
Line of credit facility, amount outstanding			$ 0		$ 0