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Fair Value Measurements - Fair Value of Assets Measured on Recurring Basis (Details) - USD ($) $ in Millions | Jun. 30, 2025 | Dec. 31, 2024
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Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | $ 8,018.9 | $ 7,265.9
Equity securities, estimated fair value | 396.0 | 386.8
Total assets | 8,414.9 | 7,652.7
U.S. Treasury bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 170.2 | 175.6
Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 876.7 | 776.5
Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 225.5 | 211.6
Governmental agency bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 206.3 | 189.8
Governmental agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 5,037.9 | 4,502.3
U.S. corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 1,010.0 | 925.6
Foreign corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 492.3 | 484.5
Common Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 384.2 | 374.7
Preferred Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 11.8 | 12.1
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Equity securities, estimated fair value | 396.0 | 386.8
Total assets | 396.0 | 386.8
Level 1 | U.S. Treasury bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Governmental agency bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Governmental agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | U.S. corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Foreign corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 1 | Common Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 384.2 | 374.7
Level 1 | Preferred Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 11.8 | 12.1
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 8,018.9 | 7,265.9
Equity securities, estimated fair value | 0.0 | 0.0
Total assets | 8,018.9 | 7,265.9
Level 2 | U.S. Treasury bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 170.2 | 175.6
Level 2 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 876.7 | 776.5
Level 2 | Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 225.5 | 211.6
Level 2 | Governmental agency bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 206.3 | 189.8
Level 2 | Governmental agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 5,037.9 | 4,502.3
Level 2 | U.S. corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 1,010.0 | 925.6
Level 2 | Foreign corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 492.3 | 484.5
Level 2 | Common Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 0.0 | 0.0
Level 2 | Preferred Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 0.0 | 0.0
Level 3 |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Equity securities, estimated fair value | 0.0 | 0.0
Total assets | 0.0 | 0.0
Level 3 | U.S. Treasury bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Municipal bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Foreign government bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Governmental agency bonds |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Governmental agency mortgage-backed securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | U.S. corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Foreign corporate debt securities |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Debt securities, estimated fair value | 0.0 | 0.0
Level 3 | Common Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | 0.0 | 0.0
Level 3 | Preferred Stock |  |
Fair Value, Assets and Liabilities Measured on Recurring Basis [Abstract] |  |
Equity securities, estimated fair value | $ 0.0 | $ 0.0

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- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1A -Publisher FASB -URI https://asc.fasb.org/1943274/2147482736/825-10-45-1A Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 210 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 1 -Subparagraph (f) -Publisher FASB -URI https://asc.fasb.org/1943274/2147483467/210-10-45-1 + Details
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