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Investments (Schedule of Unrealized Loss on Investments) (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023
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Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | $ 2.3 | $ 8.4
12 months or less, Fair Value | 693.8 | 509.8
Greater than 12 months, Gross Unrealized Losses | 394.3 | 616.3
Greater than 12 months, Fair Value | 4,721.4 | 6,101.1
Total, Gross Unrealized Losses | 396.6 | 624.7
Total, Fair Value | 5,415.2 | 6,610.9
Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 1.4 | 5.9
12 months or less, Fair Value | 632.1 | 476.0
Greater than 12 months, Gross Unrealized Losses | 389.5 | 609.9
Greater than 12 months, Fair Value | 4,663.4 | 6,034.0
Total, Gross Unrealized Losses | 390.9 | 615.8
Total, Fair Value | 5,295.5 | 6,510.0
Below Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.9 | 2.5
12 months or less, Fair Value | 61.7 | 33.8
Greater than 12 months, Gross Unrealized Losses | 4.8 | 6.4
Greater than 12 months, Fair Value | 58.0 | 67.1
Total, Gross Unrealized Losses | 5.7 | 8.9
Total, Fair Value | 119.7 | 100.9
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 | 0.3 | 0.6
12 months or less, Fair Value | 106.0 | 62.2
Greater than 12 months, Gross Unrealized Losses | 43.7 | 51.1
Greater than 12 months, Fair Value | 330.2 | 331.0
Total, Gross Unrealized Losses | 44.0 | 51.7
Total, Fair Value | 436.2 | 393.2
Foreign Governments [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Greater than 12 months, Fair Value |  | 2.0
Total, Fair Value |  | 2.0
Municipals [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.2 | 2.5
12 months or less, Fair Value | 39.8 | 72.5
Greater than 12 months, Gross Unrealized Losses | 89.8 | 127.1
Greater than 12 months, Fair Value | 848.0 | 935.2
Total, Gross Unrealized Losses | 90.0 | 129.6
Total, Fair Value | 887.8 | 1,007.7
Corporates [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.6 | 1.3
12 months or less, Fair Value | 347.4 | 159.9
Greater than 12 months, Gross Unrealized Losses | 112.6 | 214.9
Greater than 12 months, Fair Value | 2,005.6 | 2,870.2
Total, Gross Unrealized Losses | 113.2 | 216.2
Total, Fair Value | 2,353.0 | 3,030.1
Corporates [Member] | Below Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.9 | 2.5
12 months or less, Fair Value | 61.7 | 33.8
Greater than 12 months, Gross Unrealized Losses | 4.8 | 6.4
Greater than 12 months, Fair Value | 57.1 | 66.2
Total, Gross Unrealized Losses | 5.7 | 8.9
Total, Fair Value | 118.8 | 100.0
Residential Mortgage-Backed [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.1 | 1.2
12 months or less, Fair Value | 78.1 | 139.3
Greater than 12 months, Gross Unrealized Losses | 105.0 | 129.0
Greater than 12 months, Fair Value | 808.1 | 865.4
Total, Gross Unrealized Losses | 105.1 | 130.2
Total, Fair Value | 886.2 | 1,004.7
Commercial Mortgage-Backed [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses |  | 0.2
12 months or less, Fair Value |  | 14.0
Greater than 12 months, Gross Unrealized Losses | 36.4 | 72.0
Greater than 12 months, Fair Value | 545.4 | 770.0
Total, Gross Unrealized Losses | 36.4 | 72.2
Total, Fair Value | 545.4 | 784.0
Commercial Mortgage-Backed [Member] | Below Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Greater than 12 months, Fair Value | 0.9 | 0.9
Total, Fair Value | 0.9 | 0.9
Other Asset-Backed [Member] | Investment Grade [Member] |  |
Schedule of Available-for-sale Securities [Line Items] |  |
12 months or less, Gross Unrealized Losses | 0.2 | 0.1
12 months or less, Fair Value | 60.8 | 28.1
Greater than 12 months, Gross Unrealized Losses | 2.0 | 15.8
Greater than 12 months, Fair Value | 126.1 | 260.2
Total, Gross Unrealized Losses | 2.2 | 15.9
Total, Fair Value | $ 186.9 | $ 288.3

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