Debt - Narrative (Details) - USD ($)						1 Months Ended	3 Months Ended	12 Months Ended
	Jun. 07, 2019	Jun. 04, 2019	Dec. 28, 2018	Jul. 02, 2018	Jun. 29, 2018	Jun. 30, 2017	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 22, 2020	Jul. 05, 2019	Jun. 08, 2018	Feb. 28, 2015
Debt Instrument [Line Items]
Repayments of debt			$ 215,000,000.0
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)								$ 1,172,800,000	$ 778,400,000
Loss from Early Extinguishment of Debt							$ 5,800,000	0	5,800,000	$ 0
Interest expense								36,000,000.0	42,500,000	43,300,000
Interest paid including facility fees								34,600,000	44,000,000.0	$ 37,900,000
Advances from Federal Home Loan Banks								0
Secured Debt | Term Loan due July 5, 2023
Debt Instrument [Line Items]
Maximum Capacity Amount		$ 50,000,000.0
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)								49,900,000	49,900,000			$ 49,900,000
Senior Notes | 5.000% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)								278,300,000	279,900,000
Senior Notes | Senior Notes Payable, 4.35% due 2025 (Fair Value: 2020 – $499.5; 2019 – $478.6)
Debt Instrument [Line Items]
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)								$ 448,800,000	$ 448,600,000
Stated interest rate, percentage								4.35%	4.35%
Debt instrument, face amount														$ 250,000,000.0
Increase in debt instrument						$ 200,000,000.0
Long-term debt, gross								$ 450,000,000.0
Senior Notes | Senior Notes, 2.400 Percent Due September 30, 2030
Debt Instrument [Line Items]
Proceeds from debt, net of issuance costs								395,600,000
Long-term Debt, Current and Non-current, at Amortized Cost (Fair Value: 2020 - $1,247.8; 2019 - $820.2)								395,800,000	$ 0
Stated interest rate, percentage											2.40%
Debt instrument, effective interest rate											2.52%
Long-term debt, gross								$ 400,000,000.0
Subordinated Debt | Subordinated Debentures due 2054 (Fair Value: 2018 – $151.1)
Debt Instrument [Line Items]
Stated interest rate, percentage	7.375%
Long-term debt, gross	$ 150,000,000.0
Redemption price, percentage	100.00%
Revolving Credit Facility | Notes Payable under Revolving Credit Agreement | Second Amended and Restated Credit Agreement
Debt Instrument [Line Items]
Maximum Capacity Amount		400,000,000.0											$ 300,000,000.0
Increase in line of credit facility		100,000,000.0
Debt issuance costs		100,000
Unamortized debt issuance expense		$ 900,000
Line of credit, outstanding									$ 0
Infinity Property and Casualty Corporation
Debt Instrument [Line Items]
Fair value of senior notes assumed				$ 282,100,000
Infinity Property and Casualty Corporation | Secured Debt
Debt Instrument [Line Items]
Business combination, consideration transferred, liabilities incurred				$ 110,000,000.0	$ 250,000,000.0
Infinity Property and Casualty Corporation | Senior Notes | 5.000% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Stated interest rate, percentage				5.00%
Debt instrument, face amount				$ 275,000,000.0
Fair value of senior notes assumed				282,100,000
Unamortized premium				$ 7,100,000
Debt instrument, effective interest rate				4.36%