Investment Securities - Investment Holdings (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2025	Dec. 31, 2024
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 444.0	$ 461.8
Gross Unrealized Gains	2.2	1.1
Gross Unrealized Losses	(0.2)	(0.8)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	446.0	462.1
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,479.7	1,444.5
Gross Unrealized Gains	14.4	4.6
Gross Unrealized Losses	(13.2)	(31.4)
Net Foreign Currency Gains (Losses)	0.3	(3.5)
Carrying Value	1,481.2	1,414.2
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	3.2	3.2
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	0.0	0.0
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	3.3	3.2
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	411.4	400.2
Gross Unrealized Gains	3.3	0.2
Gross Unrealized Losses	(17.4)	(26.5)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	397.3	373.9
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	358.1	237.3
Gross Unrealized Gains	1.2	1.2
Gross Unrealized Losses	(0.1)	(0.2)
Net Foreign Currency Gains (Losses)	2.9	(1.6)
Carrying Value	362.1	236.7
Foreign government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	27.2	22.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.1)
Net Foreign Currency Gains (Losses)	1.3	(0.6)
Carrying Value	28.5	21.5
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	2,723.6	2,569.2
Gross Unrealized Gains	21.2	7.1
Gross Unrealized Losses	(30.9)	(59.0)
Net Foreign Currency Gains (Losses)	4.5	(5.7)
Carrying Value	$ 2,718.4	$ 2,511.6