Investment Securities - Investment Holdings Fixed Maturity Table (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2023	Mar. 31, 2022
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 245.7	$ 216.6
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(8.0)	(10.2)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	237.8	206.4
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,073.9	1,098.3
Gross Unrealized Gains	1.3	0.6
Gross Unrealized Losses	(63.1)	(78.3)
Net Foreign Currency Gains (Losses)	(1.7)	(1.8)
Carrying Value	1,010.4	1,018.8
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	280.0	281.6
Gross Unrealized Gains	0.0	0.4
Gross Unrealized Losses	(17.0)	(23.4)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	263.0	258.6
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	289.5	288.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(30.7)	(34.5)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	258.8	254.2
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	192.1	190.8
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(5.2)	(6.0)
Net Foreign Currency Gains (Losses)	(1.1)	(2.0)
Carrying Value	185.9	182.9
Total fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	2,081.2	2,076.0
Gross Unrealized Gains	1.5	1.1
Gross Unrealized Losses	(124.0)	(152.4)
Net Foreign Currency Gains (Losses)	(2.8)	(3.8)
Carrying Value	$ 1,955.9	$ 1,920.9

Amount of unrealized gain on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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-Name Accounting Standards Codification
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