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Investment Securities - Investment Holdings Fixed Maturity Table (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 227.9	$ 212.1
Gross Unrealized Gains	0.0	0.5
Gross Unrealized Losses	(11.0)	(1.1)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	216.9	211.5
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,068.8	993.3
Gross Unrealized Gains	0.0	8.7
Gross Unrealized Losses	(91.2)	(8.7)
Net Foreign Currency Gains (Losses)	(3.1)	(0.4)
Carrying Value	974.5	992.9
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	289.3	276.4
Gross Unrealized Gains	0.0	16.8
Gross Unrealized Losses	(24.4)	(1.3)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	264.9	291.9
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	278.3	277.2
Gross Unrealized Gains	0.0	2.9
Gross Unrealized Losses	(37.6)	(2.5)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	240.7	277.6
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	169.7	136.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(7.6)	(0.4)
Net Foreign Currency Gains (Losses)	(4.8)	(1.1)
Carrying Value	157.3	135.0
Total fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	2,034.0	1,895.5
Gross Unrealized Gains	0.0	28.9
Gross Unrealized Losses	(171.8)	(14.0)
Net Foreign Currency Gains (Losses)	(7.9)	(1.5)
Carrying Value	$ 1,854.3	$ 1,908.9

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