Investments Gross Unrealized Losses and Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2026	Dec. 31, 2025
Schedule of Fixed Income Securities [Table] [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	$ 3,921.9	$ 1,075.1
Available for sale, Unrealized Losses, Less than 12 Months	43.6	5.4
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	991.2	1,362.7
Available for sale, Unrealized Losses, 12 Months or Greater	34.9	32.7
Debt Securities, Available-for-sale, Unrealized Loss Position	4,913.2	2,437.8
U.S. & Canadian Governments [Member]
Schedule of Fixed Income Securities [Table] [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	747.2	179.9
Available for sale, Unrealized Losses, Less than 12 Months	9.0	1.0
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	518.7	594.7
Available for sale, Unrealized Losses, 12 Months or Greater	24.2	20.1
Debt Securities, Available-for-sale, Unrealized Loss Position	1,266.0	774.7
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	33.2	21.1
Corporate [Member]
Schedule of Fixed Income Securities [Table] [Line Items]
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, Less than 12 Months	3,174.6	895.2
Available for sale, Unrealized Losses, Less than 12 Months	34.5	4.3
Debt Securities, Available-for-sale, Continuous Unrealized Loss Position, 12 Months or Longer	472.5	767.9
Available for sale, Unrealized Losses, 12 Months or Greater	10.6	12.6
Debt Securities, Available-for-sale, Unrealized Loss Position	3,647.2	1,663.1
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	45.2	16.9
Fixed Income Securities [Member]
Schedule of Fixed Income Securities [Table] [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	$ 78.5	$ 38.1

[Line Items] for Schedule of Available-for-sale and Held-to-maturity Securities [Table] [Table]

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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