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Investments - Amortized Cost and Fair Value of Fixed Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Cost or Amortized Cost
Due in one year or less, cost or amortized cost	$ 153.0
Due after one year through five years, cost or amortized cost	1,406.7
Due after five years through ten years, cost or amortized cost	3,730.5
Due after ten years, cost or amortized cost	1,343.6
Total, cost or amortized cost	6,633.8
Fixed maturity securities available for sale, amortized cost	8,956.7	$ 8,635.3
Fair Value
Due in one year or less, fair value	152.8
Due after one year through five years, fair value	1,408.7
Due after five years through ten years, fair value	3,735.3
Due after ten years, fair value	1,216.4
Total, fair value	6,513.2
Fixed maturity securities, fair value	8,783.8	8,577.7
Commercial mortgage-backed
Cost or Amortized Cost
Cost or amortized cost	434.4
Fair Value
Fair value	419.0
Residential mortgage-backed
Cost or Amortized Cost
Cost or amortized cost	1,032.2
Fair Value
Fair value	1,001.5
Asset-backed
Cost or Amortized Cost
Cost or amortized cost	856.3
Fixed maturity securities available for sale, amortized cost	856.3	850.2
Fair Value
Fair value	850.1
Fixed maturity securities, fair value	$ 850.1	$ 845.7

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