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Acquisition of Business - Narrative (Details) $ / shares in Units, $ in Millions						12 Months Ended
	Nov. 23, 2020 USD ($)	Dec. 28, 2018 USD ($)	Jul. 13, 2018 USD ($)	Jul. 02, 2018 USD ($) $ / shares shares	Jun. 29, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) agent	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Repayments of secured debt						$ 304.8	$ 383.6	$ 2.5
Repayments of debt		$ 215.0
Goodwill						$ 1,114.0	1,114.0
Current Fiscal Year End Date						--12-31
American Access Casualty Company
Business Acquisition [Line Items]
Total consideration transferred	$ 370.0
Direct premiums written							$ 370.0
Number of independent agents \| agent							500
Number of captive agents \| agent							110
Infinity Property and Casualty Corporation
Business Acquisition [Line Items]
Total consideration transferred				$ 1,500.0
Percentage of voting interests acquired				100.00%
Business acquisition, interest issued (in shares) \| shares				13,184,107
Business acquisition, fair value of interests issued				$ 982.5
Share price \| $ / shares				$ 74.53
Cash consideration				$ 564.6
Assumed legal and tax accruals							$ 1.8
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Deferred Tax Liabilities				10.8
Increase current income taxes assets				1.0			0.2
Goodwill				791.0			$ 1.6
Secured Debt \| Infinity Property and Casualty Corporation
Business Acquisition [Line Items]
Borrowings				$ 110.0	$ 250.0
Repayments of secured debt			$ 110.0
Restricted Stock Units (RSUs)
Business Acquisition [Line Items]
Business acquisition, interest issued (in shares) \| shares				44,010
Restricted Stock Units (RSUs) \| Infinity Property and Casualty Corporation
Business Acquisition [Line Items]
Business acquisition, fair value of interests issued				$ 3.3
Consideration related to pre-close activities				1.6
Consideration related to future services				$ 1.7