Investments - Amortized Cost and Fair Value of Fixed Maturity Securities by Contractual Maturity (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Cost or Amortized Cost
Due in one year or less, cost or amortized cost	$ 202.6
Due after one year through five years, cost or amortized cost	1,488.1
Due after five years through ten years, cost or amortized cost	2,980.7
Due after ten years, cost or amortized cost	1,111.8
Total, cost or amortized cost	5,783.2
Fixed maturity securities available for sale, amortized cost	7,766.4	$ 7,292.4
Fair Value
Due in one year or less, fair value	202.1
Due after one year through five years, fair value	1,492.8
Due after five years through ten years, fair value	2,969.8
Due after ten years, fair value	1,011.8
Total, fair value	5,676.5
Fixed maturity securities, fair value	7,596.1	6,912.1
Commercial mortgage-backed
Cost or Amortized Cost
Cost or amortized cost	356.1
Fair Value
Fair value	325.2
Residential mortgage-backed
Cost or Amortized Cost
Cost or amortized cost	656.9
Fair Value
Fair value	624.1
Asset-backed
Cost or Amortized Cost
Cost or amortized cost	970.2
Fixed maturity securities available for sale, amortized cost	970.2	891.4
Fair Value
Fair value	970.3
Fixed maturity securities, fair value	$ 970.3	$ 873.8

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