Investment Securities - Fixed Maturity Investments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 2,142.6	$ 2,076.0
Gross Unrealized Gains	0.4	1.1
Gross Unrealized Losses	(160.3)	(152.4)
Net Foreign Currency Gains (Losses)	(4.8)	(3.8)
Carrying Value	1,977.9	1,920.9
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	270.1	216.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(9.0)	(10.2)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	261.1	206.4
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,038.8	1,098.3
Gross Unrealized Gains	0.1	0.6
Gross Unrealized Losses	(74.6)	(78.3)
Net Foreign Currency Gains (Losses)	(2.7)	(1.8)
Carrying Value	961.6	1,018.8
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	264.6	281.6
Gross Unrealized Gains	0.0	0.4
Gross Unrealized Losses	(26.4)	(23.4)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	238.2	258.6
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	372.6	288.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(47.2)	(34.5)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	325.4	254.2
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	196.5	190.8
Gross Unrealized Gains	0.3	0.1
Gross Unrealized Losses	(3.1)	(6.0)
Net Foreign Currency Gains (Losses)	(2.1)	(2.0)
Carrying Value	$ 191.6	$ 182.9