Investments, Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Amortized Cost
Due in one year or less, amortized cost	$ 97.4
Due after one year through five years, amortized cost	766.4
Due after five years through ten years, amortized cost	733.2
Due after ten years, amortized cost	108.1
Mortgage and asset-backed securities, amortized cost	561.0
Debt securities, available-for-sale, amortized cost	2,266.1	$ 2,333.6
Estimated Fair Value
Due in one year or less, fair value	98.8
Due after one year through five years, fair value	799.8
Due after five years through ten years, fair value	763.4
Due after ten years, fair value	112.7
Mortgage and asset-backed securities, fair value	568.0
Fixed maturity securities, fair value	$ 2,342.7	$ 2,479.2

Amortized cost of 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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