Debt - Narrative (Details) - USD ($)					1 Months Ended		3 Months Ended		9 Months Ended
	Jun. 07, 2019	Jun. 04, 2019	Dec. 28, 2018	Jun. 29, 2018	May 31, 2019	Jun. 30, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Jul. 05, 2019	Dec. 31, 2018	Jul. 02, 2018	Jun. 08, 2018	Mar. 31, 2018	Feb. 28, 2015
Debt Instrument [Line Items]
Repayments of debt			$ 215,000,000.0		$ 35,000,000.0
Debt							$ 778,700,000		$ 778,700,000			$ 909,000,000.0
Loss from Early Extinguishment of Debt							5,800,000	$ 0	5,800,000	$ 0
Collateralized Investment Borrowings							137,600,000		137,600,000			10,000,000.0
Interest and other expenses							9,200,000	13,400,000	32,500,000	29,300,000
Interest paid, including capitalized interest, operating and investing activities							18,900,000	$ 19,700,000	43,000,000.0	$ 35,000,000.0
Secured Debt
Debt Instrument [Line Items]
Proceeds from debt, net of issuance costs				$ 249,400,000
Secured Debt | 5.0% Senior Notes due September 19, 2022
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	0
Senior Notes | 5.0% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Debt												281,500,000
Senior Notes | 4.35% Senior Notes due February 15, 2025
Debt Instrument [Line Items]
Debt							$ 448,500,000		$ 448,500,000			448,400,000
Stated interest rate							4.35%		4.35%
Face amount of debt																$ 250,000,000.0
Increase in senior notes						$ 200,000,000
Subordinated Debt | 7.375% Subordinated Debentures due February 27, 2054
Debt Instrument [Line Items]
Debt							$ 0		$ 0			144,200,000
Stated interest rate	7.375%
Long-term debt, gross	$ 150,000,000.0
Redemption price, percentage	100.00%
Infinity Property and Casualty Corporation
Debt Instrument [Line Items]
Business combination, liabilities assumed, long-term debt													$ 282,100,000
Infinity Property and Casualty Corporation | Secured Debt
Debt Instrument [Line Items]
Business combination, consideration transferred, liabilities incurred				$ 250,000,000.0
Infinity Property and Casualty Corporation | Senior Notes | 5.0% Senior Notes due September 19, 2022
Debt Instrument [Line Items]
Debt							280,300,000		280,300,000
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		$ 1,100,000
Line of credit facility, amount outstanding							0		0			0
Federal Home Loan Bank of Chicago
Debt Instrument [Line Items]
Collateralized Investment Borrowings							137,600,000		137,600,000			10,000,000.0
United Insurance Company of America | Federal Home Loan Bank of Chicago
Debt Instrument [Line Items]
Debt							137,600,000		137,600,000
Federal Home Loan Bank stock							2,800,000		2,800,000			800,000
Federal Home Loan Bank, amount of advances							385,600,000		385,600,000						$ 10,000,000.0
Collateral amount							184,100,000		184,100,000			15,700,000
Repayment of collateralized investment borrowings									247,900,000
Collateralized Investment Borrowings							137,600,000		137,600,000
Trinity Universal Insurance Company | Federal Home Loan Bank of Dallas
Debt Instrument [Line Items]
Federal Home Loan Bank stock							$ 3,300,000		$ 3,300,000			$ 3,300,000