Investment Securities - Investment Holdings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 459.7	$ 461.8
Gross Unrealized Gains	1.3	1.1
Gross Unrealized Losses	(0.7)	(0.8)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	460.3	462.1
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,534.9	1,444.5
Gross Unrealized Gains	13.5	4.6
Gross Unrealized Losses	(11.5)	(31.4)
Net Foreign Currency Gains (Losses)	0.8	(3.5)
Carrying Value	1,537.7	1,414.2
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost		3.2
Gross Unrealized Gains		0.0
Gross Unrealized Losses		0.0
Net Foreign Currency Gains (Losses)		0.0
Carrying Value		3.2
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	405.1	400.2
Gross Unrealized Gains	4.2	0.2
Gross Unrealized Losses	(16.3)	(26.5)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	393.0	373.9
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	343.9	237.3
Gross Unrealized Gains	0.8	1.2
Gross Unrealized Losses	(0.1)	(0.2)
Net Foreign Currency Gains (Losses)	2.4	(1.6)
Carrying Value	347.0	236.7
Foreign government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	31.2	22.2
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.1)	(0.1)
Net Foreign Currency Gains (Losses)	1.4	(0.6)
Carrying Value	32.5	21.5
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	2,774.8	2,569.2
Gross Unrealized Gains	19.8	7.1
Gross Unrealized Losses	(28.7)	(59.0)
Net Foreign Currency Gains (Losses)	4.6	(5.7)
Carrying Value	$ 2,770.5	$ 2,511.6