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Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions | Dec. 31, 2025 | Dec. 31, 2024
--- | --- | ---
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | $ 5.3 | $ 54.6
12 months or less, Fair Value | 712.4 | 2,860.0
Greater than 12 months, Gross Unrealized Losses | 261.9 | 476.5
Greater than 12 months, Fair Value | 2,444.2 | 3,806.7
Total, Gross Unrealized Losses | 267.2 | 531.1
Total, Fair Value | 3,156.6 | 6,666.7
Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 3.1 | 53.3
12 months or less, Fair Value | 673.6 | 2,789.1
Greater than 12 months, Gross Unrealized Losses | 258.5 | 471.2
Greater than 12 months, Fair Value | 2,404.5 | 3,756.1
Total, Gross Unrealized Losses | 261.6 | 524.5
Total, Fair Value | 3,078.1 | 6,545.2
Below Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 2.2 | 1.3
12 months or less, Fair Value | 38.8 | 70.9
Greater than 12 months, Gross Unrealized Losses | 3.4 | 5.3
Greater than 12 months, Fair Value | 39.7 | 50.6
Total, Gross Unrealized Losses | 5.6 | 6.6
Total, Fair Value | 78.5 | 121.5
U.S. Treasury and Government Agencies [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.0 | 7.8
12 months or less, Fair Value | 11.9 | 194.4
Greater than 12 months, Gross Unrealized Losses | 38.2 | 51.0
Greater than 12 months, Fair Value | 319.7 | 240.8
Total, Gross Unrealized Losses | 38.2 | 58.8
Total, Fair Value | 331.6 | 435.2
Foreign Governments [Member] | Below Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses |  | 0.0
12 months or less, Fair Value |  | 0.1
Greater than 12 months, Gross Unrealized Losses |  | 0.0
Greater than 12 months, Fair Value |  | 0.0
Total, Gross Unrealized Losses |  | 0.0
Total, Fair Value |  | 0.1
Municipals [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.6 | 3.0
12 months or less, Fair Value | 56.3 | 132.5
Greater than 12 months, Gross Unrealized Losses | 66.6 | 106.2
Greater than 12 months, Fair Value | 550.3 | 680.9
Total, Gross Unrealized Losses | 67.2 | 109.2
Total, Fair Value | 606.6 | 813.4
Corporates [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 1.0 | 22.8
12 months or less, Fair Value | 271.3 | 1,360.3
Greater than 12 months, Gross Unrealized Losses | 51.2 | 132.0
Greater than 12 months, Fair Value | 704.2 | 1,523.3
Total, Gross Unrealized Losses | 52.2 | 154.8
Total, Fair Value | 975.5 | 2,883.6
Corporates [Member] | Below Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 2.2 | 1.3
12 months or less, Fair Value | 37.5 | 70.8
Greater than 12 months, Gross Unrealized Losses | 2.6 | 5.3
Greater than 12 months, Fair Value | 34.8 | 49.8
Total, Gross Unrealized Losses | 4.8 | 6.6
Total, Fair Value | 72.3 | 120.6
Residential Mortgage-Backed [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.5 | 16.5
12 months or less, Fair Value | 128.7 | 896.8
Greater than 12 months, Gross Unrealized Losses | 82.3 | 139.8
Greater than 12 months, Fair Value | 521.7 | 747.2
Total, Gross Unrealized Losses | 82.8 | 156.3
Total, Fair Value | 650.4 | 1,644.0
Commercial Mortgage-Backed [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.5 | 0.7
12 months or less, Fair Value | 122.5 | 23.7
Greater than 12 months, Gross Unrealized Losses | 20.0 | 41.4
Greater than 12 months, Fair Value | 284.3 | 485.4
Total, Gross Unrealized Losses | 20.5 | 42.1
Total, Fair Value | 406.8 | 509.1
Commercial Mortgage-Backed [Member] | Below Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.0 | 0.0
12 months or less, Fair Value | 0.0 | 0.0
Greater than 12 months, Gross Unrealized Losses | 0.8 | 0.0
Greater than 12 months, Fair Value | 4.9 | 0.8
Total, Gross Unrealized Losses | 0.8 | 0.0
Total, Fair Value | 4.9 | 0.8
Other Asset-Backed [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.5 | 2.5
12 months or less, Fair Value | 82.9 | 181.4
Greater than 12 months, Gross Unrealized Losses | 0.2 | 0.8
Greater than 12 months, Fair Value | 24.3 | 78.5
Total, Gross Unrealized Losses | 0.7 | 3.3
Total, Fair Value | 107.2 | $ 259.9
Other Asset-Backed [Member] | Below Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.0 |
12 months or less, Fair Value | 1.3 |
Greater than 12 months, Gross Unrealized Losses | 0.0 |
Greater than 12 months, Fair Value | 0.0 |
Total, Gross Unrealized Losses | 0.0 |
Total, Fair Value | $ 1.3 |

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