Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfers out of Level 3	$ 0.0	$ 211.7	$ 5.7	$ 212.3
Debt Securities, Available-for-Sale	6,577.1		6,577.1		$ 7,986.9
Equity Securities, FV-NI, Current	322.7		322.7		830.6
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	7,044.4		7,044.4		8,863.9
Convertible debt at fair value	42.6		42.6		46.4
Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	208.5		208.5		325.0
Convertible debt at fair value	0.0		0.0		0.0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	233.2		233.2		251.4
Convertible debt at fair value	0.0		0.0		0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	183.8		183.8		586.6
Convertible debt at fair value	0.0		0.0		0.0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	6,418.9		6,418.9		7,700.9
Convertible debt at fair value	42.6		42.6		46.4
Limited Liability Companies and Limited Partnerships | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	208.5		208.5		325.0
Limited Liability Companies and Limited Partnerships | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	208.5		208.5		325.0
Limited Liability Companies and Limited Partnerships | Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unfunded commitments	92.5		92.5
Limited Liability Companies and Limited Partnerships | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Limited Liability Companies and Limited Partnerships | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Limited Liability Companies and Limited Partnerships | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
U.S. Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	526.8		526.8		637.4
U.S. Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
U.S. Government and Government Agencies and Authorities | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
U.S. Government and Government Agencies and Authorities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	112.0		112.0		132.8
U.S. Government and Government Agencies and Authorities | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	414.8		414.8		504.6
States and Political Subdivisions | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	1,552.1		1,552.1		1,890.1
States and Political Subdivisions | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
States and Political Subdivisions | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
States and Political Subdivisions | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
States and Political Subdivisions | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	1,552.1		1,552.1		1,890.1
Foreign Governments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	3.7		3.7		5.5
Foreign Governments | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Foreign Governments | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Foreign Governments | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Foreign Governments | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	3.7		3.7		5.5
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	3,341.5		3,341.5		4,386.9
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Corporate Debt Securities [Member] | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	218.7		218.7		236.8
Corporate Debt Securities [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Corporate Debt Securities [Member] | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	3,122.8		3,122.8		4,150.1
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	8.0		8.0		7.4
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0
Redeemable Preferred Stocks | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	6.8		6.8
Redeemable Preferred Stocks | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0
Redeemable Preferred Stocks | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	1.2		1.2
Collateralized Loan Obligations | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	952.6		952.6		752.1
Collateralized Loan Obligations | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Collateralized Loan Obligations | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Collateralized Loan Obligations | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Collateralized Loan Obligations | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	952.6		952.6		752.1
Other Mortgage- and Asset-backed | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	294.4		294.4		307.5
Other Mortgage- and Asset-backed | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Other Mortgage- and Asset-backed | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	5.2		5.2		7.0
Other Mortgage- and Asset-backed | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Other Mortgage- and Asset-backed | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	289.2		289.2		300.5
Investments in Fixed Maturities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	6,679.1		6,679.1		7,986.9
Investments in Fixed Maturities | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0		0.0		0.0
Investments in Fixed Maturities | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	230.7		230.7		249.9
Investments in Fixed Maturities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	112.0		112.0		132.8
Investments in Fixed Maturities | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	6,336.4		6,336.4		7,604.2
Preferred Stocks | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	29.8		29.8		34.2
Preferred Stocks | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Preferred Stocks | Fair Value, Measurements, Recurring | Other Industries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	11.7		11.7		17.6
Preferred Stocks | Fair Value, Measurements, Recurring | Other Industries | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Preferred Stocks | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Preferred Stocks | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Other Industries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	2.0		2.0		1.5
Preferred Stocks | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Preferred Stocks | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Other Industries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Preferred Stocks | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	29.8		29.8		34.2
Preferred Stocks | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Other Industries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	9.7		9.7		16.1
Common Stock | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.9		0.9		18.9
Common Stock | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Common Stock | Fair Value, Measurements, Recurring | Other Industries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	1.1		1.1		2.9
Common Stock | Fair Value, Measurements, Recurring | Other Industries | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Common Stock | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Common Stock | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Other Industries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.5		0.5		0.0
Common Stock | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.9		0.9		18.9
Common Stock | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring | Other Industries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.2		0.2		2.9
Common Stock | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Common Stock | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Other Industries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.4		0.4		0.0
Exchange Traded Funds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	70.7		70.7		432.0
Exchange Traded Funds | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Exchange Traded Funds | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Exchange Traded Funds | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	70.7		70.7		432.0
Exchange Traded Funds | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0		0.0		0.0
Equity Securities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	322.7		322.7		830.6
Equity Securities | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	208.5		208.5		325.0
Equity Securities | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	2.5		2.5		1.5
Equity Securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	71.8		71.8		453.8
Equity Securities | Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	$ 39.9		$ 39.9		$ 50.3