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Investments Fixed Income Securities by Type (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Schedule of Fixed Income Securities [Table] [Line Items]
Fixed income securities (amortized cost)	$ 12,210.9
Fixed income securities (estimated fair value)	12,357.2	$ 12,091.5
Debt Securities, Available-for-sale, Allowance for Credit Loss	1.6	1.6
U.S. & Canadian Governments [Member]
Schedule of Fixed Income Securities [Table] [Line Items]
Fixed income securities (amortized cost)	1,839.2	1,950.5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	9.8	2.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	30.7	61.3
Fixed income securities (estimated fair value)	1,818.3	1,891.9
Municipal [Member]
Schedule of Fixed Income Securities [Table] [Line Items]
Fixed income securities (amortized cost)	184.2	319.6
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	0.0	0.0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0.3	1.6
Fixed income securities (estimated fair value)	183.8	317.9
Corporate [Member]
Schedule of Fixed Income Securities [Table] [Line Items]
Fixed income securities (amortized cost)	10,187.4	9,905.8
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	198.0	76.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	30.4	100.8
Fixed income securities (estimated fair value)	10,355.0	9,881.7
Fixed Income Securities [Member]
Schedule of Fixed Income Securities [Table] [Line Items]
Fixed income securities (amortized cost)	12,210.9	12,175.9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	207.9	79.4
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	61.6	163.8
Fixed income securities (estimated fair value)	$ 12,357.2	$ 12,091.5

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