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Debt - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended		9 Months Ended
Debt - Narrative (Details) - USD ($)	Jun. 04, 2019	Jun. 30, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 22, 2020	Dec. 31, 2019	Jul. 05, 2019	Jul. 02, 2018	Jun. 08, 2018	Feb. 28, 2015
Debt Instrument [Line Items]
Long-term Debt			$ 1,173,000,000.0		$ 1,173,000,000.0			$ 778,400,000
Interest and other expenses			8,300,000	$ 9,200,000	24,700,000	$ 32,500,000
Interest paid, including capitalized interest, operating and investing activities			16,900,000	$ 18,900,000	34,400,000	$ 43,000,000.0
Secured Debt \| Term Loan due July 5, 2023
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 50,000,000.0
Long-term Debt			49,900,000		49,900,000			49,900,000	$ 49,900,000
Senior Notes
Debt Instrument [Line Items]
Long-term Debt								448,600,000
Senior Notes \| 5.000% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Long-term Debt			278,700,000		278,700,000			279,900,000
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										$ 275,000,000.0
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			$ 448,800,000		$ 448,800,000
Stated interest rate			4.35%		4.35%
Long-term debt, gross			$ 450,000,000.0		$ 450,000,000.0
Face amount of debt												$ 250,000,000.0
Increase in senior notes		$ 200,000,000.0
Senior Notes \| 2.400% Senior Notes due September 30, 2030
Debt Instrument [Line Items]
Long-term Debt			395,600,000		395,600,000			0
Stated interest rate							2.40%
Long-term debt, gross			400,000,000.0		400,000,000.0
Effective interest rate							2.52%
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	400,000,000.0										$ 300,000,000.0
Increase in revolving credit borrowing capacity	100,000,000.0
Debt issuance costs	100,000
Remaining unamortized costs	$ 1,000,000.0
Line of credit facility, amount outstanding			$ 0		$ 0			$ 0