Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unfunded commitments		$ 195.7
Transfers out of Level 3	$ 13.1
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	7,002.9	7,122.4
Derivative Asset		14.7
Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	194.1	191.4
Derivative Asset		0.0
Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	191.9	192.3
Derivative Asset		0.0
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	95.0	107.3
Derivative Asset		0.0
Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	6,521.9	6,631.4
Derivative Asset		14.7
Partnership Interest [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	194.1	191.4
Partnership Interest [Member] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	194.1	191.4
Partnership Interest [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unfunded commitments	193.9
Partnership Interest [Member] | Fair Value, Inputs, Level 3 [Member] | 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
Partnership Interest [Member] | Fair Value, Inputs, Level 1 [Member] | 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
Partnership Interest [Member] | Fair Value, Inputs, Level 2 [Member] | 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
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	488.4	511.5
U.S. Government and Government Agencies and Authorities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0	0.0
U.S. Government and Government Agencies and Authorities | Fair Value, Inputs, Level 3 [Member] | 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
U.S. Government and Government Agencies and Authorities | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	85.4	98.8
U.S. Government and Government Agencies and Authorities | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	403.0	412.7
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	1,355.9	1,401.9
States and Political Subdivisions | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0	0.0
States and Political Subdivisions | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	1.4	0.1
States and Political Subdivisions | Fair Value, Inputs, Level 1 [Member] | 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
States and Political Subdivisions | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	1,354.5	1,401.8
Foreign Governments | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	4.6	3.8
Foreign Governments | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0	0.0
Foreign Governments | Fair Value, Inputs, Level 3 [Member] | 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
Foreign Governments | Fair Value, Inputs, Level 1 [Member] | 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
Foreign Governments | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	4.6	3.8
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	3,635.6	3,690.8
Corporate Debt Securities [Member] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0	0.0
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	180.5	177.1
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member] | 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
Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	3,455.1	3,513.7
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	8.3
Redeemable Preferred Stocks | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0
Redeemable Preferred Stocks | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	2.2
Redeemable Preferred Stocks | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0
Redeemable Preferred Stocks | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	5.8
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	938.2	949.8
Collateralized Loan Obligations | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0	0.0
Collateralized Loan Obligations | Fair Value, Inputs, Level 3 [Member] | 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
Collateralized Loan Obligations | Fair Value, Inputs, Level 1 [Member] | 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
Collateralized Loan Obligations | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	938.2	949.8
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	340.9	315.8
Other Mortgage- and Asset-backed | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0	0.0
Other Mortgage- and Asset-backed | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	5.1	5.2
Other Mortgage- and Asset-backed | Fair Value, Inputs, Level 1 [Member] | 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
Other Mortgage- and Asset-backed | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	335.8	310.6
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	6,771.6	6,881.9
Investments in Fixed Maturities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	0.0	0.0
Investments in Fixed Maturities | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	189.2	189.5
Investments in Fixed Maturities | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	85.4	98.8
Investments in Fixed Maturities | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt Securities, Available-for-Sale	6,497.0	6,593.6
Preferred Stocks | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	15.9	15.6
Preferred Stocks | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Preferred Stocks | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	10.0	9.9
Preferred Stocks | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Preferred Stocks | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Preferred Stocks | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	2.2	2.4
Preferred Stocks | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Preferred Stocks | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Preferred Stocks | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	15.9	15.6
Preferred Stocks | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	7.8	7.5
Common Stock [Member] | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.6	0.6
Common Stock [Member] | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Common Stock [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.8	0.6
Common Stock [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member] | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Common Stock [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Common Stock [Member] | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.5	0.4
Common Stock [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.6	0.6
Common Stock [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.3	0.2
Common Stock [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring | Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Common Stock [Member] | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring | All Industries Other than Finance, Insurance and Real Estate [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Exchange Traded Funds [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	8.7	7.7
Exchange Traded Funds [Member] | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	0.0	0.0
Exchange Traded Funds [Member] | Fair Value, Inputs, Level 3 [Member] | 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
Exchange Traded Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	8.7	7.7
Exchange Traded Funds [Member] | Fair Value, Inputs, Level 2 [Member] | 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
Equity Securities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	230.1	225.8
Equity Securities | Fair Value, Measurements, Recurring | Fair Value, Inputs, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	194.1	191.4
Equity Securities | Fair Value, Inputs, Level 3 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	2.7	2.8
Equity Securities | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	9.6	8.5
Equity Securities | Fair Value, Inputs, Level 2 [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity Securities, FV-NI, Current	$ 23.7	$ 23.1