Acquisition of Business - Narrative (Details)		12 Months Ended
	Nov. 23, 2020 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) agent	Dec. 31, 2019 USD ($)	Apr. 01, 2021 USD ($)
Business Acquisition [Line Items]
Repayments of secured debt		$ 394,000,000.0	$ 304,800,000	$ 383,600,000
Goodwill		1,312,000,000	1,114,000,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net					$ 370,900,000
Business Acquisition, Pro Forma Revenue		5,884,200,000	5,585,700,000
Business Acquisition, Pro Forma Total Expenses		6,109,500,000	5,040,100,000
Business Acquisition, Pro Forma Net Income (Loss)		(225,300,000)	545,600,000
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax		$ (105,100,000)	443,000,000.0
License
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Indefinite-Lived Intangible Assets					2,500,000
Customer Relationships
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles					4,800,000
Agent Relationships
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles					7,200,000
Software Development
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles					6,500,000
Trade Names
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles					1,800,000
Property and Casualty Insurance Reserves
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities					(211,100,000)
Unearned Premium
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities					(177,800,000)
Deferred Income Tax Liabilities
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities					(7,800,000)
Other Liabilities
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities					(51,000,000.0)
Interest on Fixed Income Securities
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities					151,200,000
Dividends on Equity Securities Excluding Alternative Investments
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities					82,400,000
Short-term Investments
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Marketable Securities					100,100,000
Cash
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets					54,300,000
Premiums Receivable
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets					148,900,000
Receivables Other than Premium Receivable
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets					2,000,000.0
Goodwill
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets					198,000,000.0
Current Income Tax Asset
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets					300,000
Other Assets
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets					81,400,000
American Access Casualty Company
Business Acquisition [Line Items]
Total consideration transferred	$ 370,900,000
Direct Premiums Written			$ 350,000,000
Present Value of Future Insurance Profits, Net					$ 42,900,000
Number Of Independent Agents | agent			600