Investment Securities - Fixed Maturity Investments (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2024	Dec. 31, 2023
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 2,434.1	$ 2,198.4
Gross Unrealized Gains	27.4	9.2
Gross Unrealized Losses	(41.5)	(97.1)
Net Foreign Currency Gains (Losses)	2.3	(1.2)
Carrying Value	2,422.3	2,109.3
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	446.0	209.0
Gross Unrealized Gains	4.8	0.3
Gross Unrealized Losses	(0.6)	(4.4)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	450.2	204.9
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,318.3	1,085.9
Gross Unrealized Gains	16.1	5.6
Gross Unrealized Losses	(24.4)	(45.4)
Net Foreign Currency Gains (Losses)	0.9	(1.1)
Carrying Value	1,310.9	1,045.0
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	3.2	275.1
Gross Unrealized Gains	0.1	0.9
Gross Unrealized Losses	0.0	(15.1)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	3.3	260.9
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	379.0	417.2
Gross Unrealized Gains	5.2	2.0
Gross Unrealized Losses	(16.1)	(29.8)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	368.1	389.4
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	287.6	211.2
Gross Unrealized Gains	1.2	0.4
Gross Unrealized Losses	(0.4)	(2.4)
Net Foreign Currency Gains (Losses)	1.4	(0.1)
Carrying Value	$ 289.8	$ 209.1