Debt - Narrative (Details) - USD ($)			1 Months Ended	3 Months Ended		9 Months Ended
	Mar. 16, 2021	Jun. 04, 2019	Mar. 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Sep. 22, 2020	Jul. 05, 2019	Jul. 02, 2018	Jun. 08, 2018	Feb. 28, 2015
Debt Instrument [Line Items]
Long-term Debt				$ 1,122,100,000		$ 1,122,100,000		$ 1,172,800,000
Interest and other expenses				10,700,000	$ 8,300,000	33,000,000	$ 24,700,000
Interest paid, including capitalized interest, operating and investing activities				21,600,000	$ 16,900,000	43,700,000	$ 34,400,000
2.400% Senior Notes due September 30, 2030
Debt Instrument [Line Items]
Long-term Debt				395,800,000		395,800,000
Secured Debt
Debt Instrument [Line Items]
Repayments of Long-term Debt	$ 50,000,000
Secured Debt | Term Loan due July 5, 2023
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity		$ 50,000,000
Long-term Debt				0		0		49,900,000		$ 49,900,000
Senior Notes
Debt Instrument [Line Items]
Long-term Debt								448,800,000
Senior Notes | 5.000% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Long-term Debt				277,100,000		277,100,000		278,300,000
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				$ 448,900,000		$ 448,900,000
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				396,100,000		396,100,000		$ 395,800,000
Stated interest rate									2.40%
Long-term debt, gross				400,000,000		400,000,000
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										$ 300,000,000
Increase in revolving credit borrowing capacity		100,000,000
Debt issuance costs		100,000
Remaining unamortized costs		$ 800,000
Line of credit facility, amount outstanding				$ 0		$ 0