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Debt - Narrative (Details) - USD ($)		1 Months Ended	3 Months Ended		6 Months Ended
Debt - Narrative (Details) - USD ($)	Jun. 04, 2019	Jun. 30, 2017	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Jul. 05, 2019	Jul. 02, 2018	Jun. 08, 2018	Feb. 28, 2015
Debt Instrument [Line Items]
Debt			$ 777,700,000		$ 777,700,000		$ 778,400,000
Interest and other expenses			8,900,000	$ 11,800,000	16,400,000	$ 23,300,000
Interest paid, including capitalized interest, operating and investing activities			400,000	$ 4,300,000	17,500,000	$ 24,100,000
Secured Debt \| Term Loan due July 5, 2023
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	$ 50,000,000.0
Debt			49,900,000		49,900,000		49,900,000	$ 49,900,000
Senior Notes \| 5.0% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Debt			279,100,000		279,100,000		279,900,000
Senior Notes \| 4.35% Senior Notes due February 15, 2025
Debt Instrument [Line Items]
Debt			$ 448,700,000		$ 448,700,000		448,600,000
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
Infinity Property and Casualty Corporation \| Senior Notes \| 5.0% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Stated interest rate									5.00%
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%
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