Fair Value Measurements - Level 3 Inputs Reconciliation (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	$ 251.4	$ 449.2
Total Losses:
Included in Consolidated Statements of (Loss) Income	(12.7)	2.8
Included in Other Comprehensive (Loss) Income	(22.6)	1.4
Purchases	112.5	140.2
Settlements	0.0	(0.1)
Sales	(116.0)	(138.3)
Transfers into Level 3	23.1	19.8
Transfers out of Level 3	(5.7)	(223.6)
Balance at End of Year	$ 230.0	251.4
Fair Value Recurring Basis Unobservable Input Reconciliation, Asset, Gain (Loss), Statement Of Income, Extensible List, Not Disclosed Flag	Included in Consolidated Statements of (Loss) Income
Fair Value, Recurring Basis Unobservable Input Reconciliation, Asset, Gain (Loss) Statement Of Other Comprehensive Income. Extensible List. Not Disclosed Flag	Included in Other Comprehensive (Loss) Income
Fair Value	$ 6,894.8	7,986.9
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	243.2	830.6
Other Short-term Investments, Fair Value Disclosure	278.4	284.1
Policyholder Obligations	701.3	504.0
FHLB of Chicago | FHLB Funding Agreements | United Insurance
Total Losses:
Policyholder Obligations	601.0	401.9
Redeemable Preferred Stocks
Total Losses:
Fair Value	8.0	7.4
United States Government and Government Agencies and Authorities
Total Losses:
Fair Value	528.0	637.4
States and Political Subdivisions
Total Losses:
Fair Value	1,568.9	1,890.1
Foreign Governments
Total Losses:
Fair Value	4.1	5.5
Corporate Bonds and Notes
Total Losses:
Fair Value	3,539.4	4,386.9
Collateralized Loan Obligations
Total Losses:
Fair Value	953.9	752.1
Other Mortgage- and Asset-backed
Total Losses:
Fair Value	292.5	307.5
Fair Value, Recurring
Total Losses:
Investments, Fair Value Disclosure	7,183.0	8,863.9
Liabilities, Fair Value Disclosure	0.4	0.0
Convertible Securities at Fair Value	43.3	46.4
Fair Value, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Investments, Fair Value Disclosure	117.0	586.6
Liabilities, Fair Value Disclosure	0.0	0.0
Convertible Securities at Fair Value	0.0	0.0
Fair Value, Recurring | Significant Other Observable Inputs (Level 2)
Total Losses:
Investments, Fair Value Disclosure	6,646.9	7,700.9
Derivative Instruments Designated as Cash Flow Hedges	0.4
Liabilities, Fair Value Disclosure	0.4	0.0
Convertible Securities at Fair Value	43.3	46.4
Fair Value, Recurring | Significant Unobservable Inputs (Level 3)
Total Losses:
Investments, Fair Value Disclosure	230.0	251.4
Liabilities, Fair Value Disclosure	0.0	0.0
Convertible Securities at Fair Value	0.0	0.0
Fair Value, Recurring | Measured at Net Asset Value
Total Losses:
Investments, Fair Value Disclosure	189.1	325.0
Liabilities, Fair Value Disclosure	0.0	0.0
Convertible Securities at Fair Value	0.0	0.0
Fair Value, Recurring | Redeemable Preferred Stocks
Total Losses:
Fair Value	8.0
Fair Value, Recurring | Redeemable Preferred Stocks | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Fair Value		0.0
Fair Value, Recurring | Redeemable Preferred Stocks | Significant Other Observable Inputs (Level 2)
Total Losses:
Fair Value		1.3
Fair Value, Recurring | Redeemable Preferred Stocks | Significant Unobservable Inputs (Level 3)
Total Losses:
Fair Value		6.1
Fair Value, Recurring | Redeemable Preferred Stocks | Measured at Net Asset Value
Total Losses:
Fair Value		0.0
Fair Value, Recurring | United States Government and Government Agencies and Authorities
Total Losses:
Fair Value	528.0	637.4
Fair Value, Recurring | United States Government and Government Agencies and Authorities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Fair Value	103.6	132.8
Fair Value, Recurring | United States Government and Government Agencies and Authorities | Significant Other Observable Inputs (Level 2)
Total Losses:
Fair Value	424.4	504.6
Fair Value, Recurring | United States Government and Government Agencies and Authorities | Significant Unobservable Inputs (Level 3)
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | United States Government and Government Agencies and Authorities | Measured at Net Asset Value
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | States and Political Subdivisions
Total Losses:
Fair Value	1,568.9	1,890.1
Fair Value, Recurring | States and Political Subdivisions | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | States and Political Subdivisions | Significant Other Observable Inputs (Level 2)
Total Losses:
Fair Value	1,568.9	1,890.1
Fair Value, Recurring | States and Political Subdivisions | Significant Unobservable Inputs (Level 3)
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | States and Political Subdivisions | Measured at Net Asset Value
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Foreign Governments
Total Losses:
Fair Value	4.1	5.5
Fair Value, Recurring | Foreign Governments | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Foreign Governments | Significant Other Observable Inputs (Level 2)
Total Losses:
Fair Value	4.1	5.5
Fair Value, Recurring | Foreign Governments | Significant Unobservable Inputs (Level 3)
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Foreign Governments | Measured at Net Asset Value
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Corporate Bonds and Notes
Total Losses:
Fair Value	3,539.4	4,386.9
Fair Value, Recurring | Corporate Bonds and Notes | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Corporate Bonds and Notes | Significant Other Observable Inputs (Level 2)
Total Losses:
Fair Value	3,323.4	4,150.1
Fair Value, Recurring | Corporate Bonds and Notes | Significant Unobservable Inputs (Level 3)
Total Losses:
Fair Value	216.0	236.8
Fair Value, Recurring | Corporate Bonds and Notes | Measured at Net Asset Value
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Redeemable Preferred Stock
Total Losses:
Fair Value		7.4
Fair Value, Recurring | Redeemable Preferred Stock | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Fair Value	0.0
Fair Value, Recurring | Redeemable Preferred Stock | Significant Other Observable Inputs (Level 2)
Total Losses:
Fair Value	1.2
Fair Value, Recurring | Redeemable Preferred Stock | Significant Unobservable Inputs (Level 3)
Total Losses:
Fair Value	6.8
Fair Value, Recurring | Redeemable Preferred Stock | Measured at Net Asset Value
Total Losses:
Fair Value	0.0
Fair Value, Recurring | Collateralized Loan Obligations
Total Losses:
Fair Value	953.9	752.1
Fair Value, Recurring | Collateralized Loan Obligations | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Collateralized Loan Obligations | Significant Other Observable Inputs (Level 2)
Total Losses:
Fair Value	953.9	752.1
Fair Value, Recurring | Collateralized Loan Obligations | Significant Unobservable Inputs (Level 3)
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Collateralized Loan Obligations | Measured at Net Asset Value
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Other Mortgage- and Asset-backed
Total Losses:
Fair Value	292.5	307.5
Fair Value, Recurring | Other Mortgage- and Asset-backed | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Other Mortgage- and Asset-backed | Significant Other Observable Inputs (Level 2)
Total Losses:
Fair Value	287.4	300.5
Fair Value, Recurring | Other Mortgage- and Asset-backed | Significant Unobservable Inputs (Level 3)
Total Losses:
Fair Value	5.1	7.0
Fair Value, Recurring | Other Mortgage- and Asset-backed | Measured at Net Asset Value
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Investments in Fixed Maturities
Total Losses:
Fair Value	6,894.8	7,986.9
Fair Value, Recurring | Investments in Fixed Maturities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Fair Value	103.6	132.8
Fair Value, Recurring | Investments in Fixed Maturities | Significant Other Observable Inputs (Level 2)
Total Losses:
Fair Value	6,563.3	7,604.2
Fair Value, Recurring | Investments in Fixed Maturities | Significant Unobservable Inputs (Level 3)
Total Losses:
Fair Value	227.9	249.9
Fair Value, Recurring | Investments in Fixed Maturities | Measured at Net Asset Value
Total Losses:
Fair Value	0.0	0.0
Fair Value, Recurring | Preferred Stocks | Finance, Insurance and Real Estate
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	29.0	34.2
Fair Value, Recurring | Preferred Stocks | Other Industries
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	10.8	17.6
Fair Value, Recurring | Preferred Stocks | Quoted Prices in Active Markets for Identical Assets (Level 1) | Finance, Insurance and Real Estate
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Preferred Stocks | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Industries
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Preferred Stocks | Significant Other Observable Inputs (Level 2) | Finance, Insurance and Real Estate
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	29.0	34.2
Fair Value, Recurring | Preferred Stocks | Significant Other Observable Inputs (Level 2) | Other Industries
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	9.2	16.1
Fair Value, Recurring | Preferred Stocks | Significant Unobservable Inputs (Level 3) | Finance, Insurance and Real Estate
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Preferred Stocks | Significant Unobservable Inputs (Level 3) | Other Industries
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	1.6	1.5
Fair Value, Recurring | Preferred Stocks | Measured at Net Asset Value | Finance, Insurance and Real Estate
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Preferred Stocks | Measured at Net Asset Value | Other Industries
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Common Stock | Finance, Insurance and Real Estate
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.9	18.9
Fair Value, Recurring | Common Stock | Other Industries
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	1.2	2.9
Fair Value, Recurring | Common Stock | Quoted Prices in Active Markets for Identical Assets (Level 1) | Finance, Insurance and Real Estate
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.9	18.9
Fair Value, Recurring | Common Stock | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other Industries
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.3	2.9
Fair Value, Recurring | Common Stock | Significant Other Observable Inputs (Level 2) | Finance, Insurance and Real Estate
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Common Stock | Significant Other Observable Inputs (Level 2) | Other Industries
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.4	0.0
Fair Value, Recurring | Common Stock | Significant Unobservable Inputs (Level 3) | Finance, Insurance and Real Estate
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Common Stock | Significant Unobservable Inputs (Level 3) | Other Industries
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.5	0.0
Fair Value, Recurring | Common Stock | Measured at Net Asset Value | Finance, Insurance and Real Estate
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Common Stock | Measured at Net Asset Value | Other Industries
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Bond Exchange Traded Funds
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	12.2	432.0
Fair Value, Recurring | Bond Exchange Traded Funds | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	12.2	432.0
Fair Value, Recurring | Bond Exchange Traded Funds | Significant Other Observable Inputs (Level 2)
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0
Fair Value, Recurring | Bond Exchange Traded Funds | Significant Unobservable Inputs (Level 3)
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0
Fair Value, Recurring | Bond Exchange Traded Funds | Measured at Net Asset Value
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Limited Liability Companies and Limited Partnerships
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	189.1	325.0
Fair Value, Recurring | Limited Liability Companies and Limited Partnerships | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Limited Liability Companies and Limited Partnerships | Significant Other Observable Inputs (Level 2)
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Limited Liability Companies and Limited Partnerships | Significant Unobservable Inputs (Level 3)
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	0.0	0.0
Fair Value, Recurring | Limited Liability Companies and Limited Partnerships | Measured at Net Asset Value
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	189.1	325.0
Fair Value, Recurring | Dividends on Equity Securities Excluding Alternative Investments
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	243.2	830.6
Fair Value, Recurring | Dividends on Equity Securities Excluding Alternative Investments | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	13.4	453.8
Fair Value, Recurring | Dividends on Equity Securities Excluding Alternative Investments | Significant Other Observable Inputs (Level 2)
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	38.6	50.3
Fair Value, Recurring | Dividends on Equity Securities Excluding Alternative Investments | Significant Unobservable Inputs (Level 3)
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	2.1	1.5
Fair Value, Recurring | Dividends on Equity Securities Excluding Alternative Investments | Measured at Net Asset Value
Total Losses:
Equity Securities at Fair Value (Cost: 2022 - $247.6; 2021 - $618.7)	189.1	325.0
Fair Value, Recurring | Designated as Hedging Instrument
Total Losses:
Derivative Instruments Designated as Cash Flow Hedges	0.4	0.0
Fair Value, Recurring | Designated as Hedging Instrument | Quoted Prices in Active Markets for Identical Assets (Level 1)
Total Losses:
Derivative Instruments Designated as Cash Flow Hedges	0.0	0.0
Fair Value, Recurring | Designated as Hedging Instrument | Significant Other Observable Inputs (Level 2)
Total Losses:
Derivative Instruments Designated as Cash Flow Hedges		0.0
Fair Value, Recurring | Designated as Hedging Instrument | Significant Unobservable Inputs (Level 3)
Total Losses:
Derivative Instruments Designated as Cash Flow Hedges	0.0	0.0
Fair Value, Recurring | Designated as Hedging Instrument | Measured at Net Asset Value
Total Losses:
Derivative Instruments Designated as Cash Flow Hedges	0.0	0.0
Corporate Bonds and Notes
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	236.8	433.0
Total Losses:
Included in Consolidated Statements of (Loss) Income	(12.7)	2.8
Included in Other Comprehensive (Loss) Income	(19.2)	1.2
Purchases	107.8	104.6
Settlements	0.0	0.0
Sales	(114.1)	(128.1)
Transfers into Level 3	23.1	8.1
Transfers out of Level 3	(5.7)	(184.8)
Balance at End of Year	216.0	236.8
Redeemable Preferred Stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	6.1	6.2
Total Losses:
Included in Consolidated Statements of (Loss) Income	0.0	0.0
Included in Other Comprehensive (Loss) Income	(1.3)	(0.1)
Purchases	2.0	0.0
Settlements	0.0	0.0
Sales	0.0	0.0
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	0.0
Balance at End of Year	6.8	6.1
Collateralized Loan Obligations
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	0.0	0.0
Total Losses:
Included in Consolidated Statements of (Loss) Income		0.0
Included in Other Comprehensive (Loss) Income		0.1
Purchases		17.7
Settlements		0.0
Sales		(10.0)
Transfers into Level 3		10.0
Transfers out of Level 3		(17.8)
Balance at End of Year		0.0
Other Mortgage- and Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	7.0	10.0
Total Losses:
Included in Consolidated Statements of (Loss) Income	0.0	0.0
Included in Other Comprehensive (Loss) Income	(1.9)	(0.5)
Purchases	0.0	16.2
Settlements	0.0	(0.1)
Sales	0.0	(0.2)
Transfers into Level 3	0.0	0.0
Transfers out of Level 3	0.0	(18.4)
Balance at End of Year	5.1	7.0
Dividends on Equity Securities Excluding Alternative Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at Beginning of Year	1.5	0.0
Total Losses:
Included in Consolidated Statements of (Loss) Income	0.0	0.0
Included in Other Comprehensive (Loss) Income	(0.2)	0.7
Purchases	2.7	1.7
Settlements	0.0	0.0
Sales	(1.9)	0.0
Transfers into Level 3	0.0	1.7
Transfers out of Level 3	0.0	(2.6)
Balance at End of Year	$ 2.1	$ 1.5