Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unfunded commitments					$ 192.2
Transfers out of Level 3	$ 0.0	$ 0.0	$ 0.1	$ 5.7
Debt Securities, Available-for-Sale	6,474.5		6,474.5		6,894.8
Equity Securities, FV-NI, Current	233.4		233.4		243.2
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets	6,753.8		6,753.8		7,183.0
Convertible debt at fair value	45.9		45.9		43.3
Derivative Asset					1.7
Total Liabilities	15.6		15.6		(0.4)
Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets	193.5		193.5		189.1
Convertible debt at fair value	0.0		0.0		0.0
Derivative Asset					0.0
Total Liabilities	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 Assets	197.6		197.6		230.0
Convertible debt at fair value	0.0		0.0		0.0
Derivative Asset					0.0
Total Liabilities	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 Assets	102.3		102.3		117.0
Convertible debt at fair value	0.0		0.0
Derivative Asset					0.0
Total Liabilities	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 Assets	6,260.4		6,260.4		6,646.9
Convertible debt at fair value	45.9		45.9		43.3
Derivative Asset					1.7
Derivative Liability	15.6		15.6		(0.4)
Total Liabilities	15.6		15.6		0.4
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	193.5		193.5		189.1
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	193.5		193.5		189.1
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	192.7		192.7
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	487.2		487.2		528.0
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	94.4		94.4		103.6
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	392.8		392.8		424.4
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	1,320.2		1,320.2		1,568.9
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	0.1		0.1		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	1,320.1		1,320.1		1,568.9
Foreign Governments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	4.2		4.2		4.1
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	4.2		4.2		4.1
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	3,394.4		3,394.4		3,539.4
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	182.4		182.4		216.0
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	3,212.0		3,212.0		3,323.4
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	7.8		7.8		8.0
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	6.6		6.6
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	956.2		956.2		953.9
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	956.2		956.2		953.9
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	304.5		304.5		292.5
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	4.8		4.8		5.1
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	299.7		299.7		287.4
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	6,474.5		6,474.5		6,894.8
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	193.9		193.9		227.9
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	94.4		94.4		103.6
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	6,186.2		6,186.2		6,563.3
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	19.9		19.9		29.0
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		10.8
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	3.3		3.3		1.6
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	19.9		19.9		29.0
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	8.4		8.4		9.2
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	0.6		0.6		0.9
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	0.6		0.6		1.2
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	0.4		0.4		0.5
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	0.6		0.6		0.9
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		0.3
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	0.0		0.0		0.4
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	7.1		7.1		12.2
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	7.1		7.1		12.2
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	233.4		233.4		243.2
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	193.5		193.5		189.1
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	3.7		3.7		2.1
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	7.9		7.9		13.4
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	28.3		28.3		38.6
Designated as Hedging Instrument | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset					(0.4)
Derivative Liability	(15.6)		(15.6)
Designated as Hedging Instrument | Fair Value, Measurements, Recurring | Measured at Net Asset Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0.0		0.0		0.0
Designated as Hedging Instrument | Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Liability	0.0		0.0		0.0
Designated as Hedging Instrument | 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]
Derivative Liability	$ 0.0		$ 0.0		$ 0.0