Investment Securities - Investment Holdings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
U.S. Government and agency obligations
Investment securities
Cost or Amortized Cost	$ 461.8	$ 209.0
Gross Unrealized Gains	1.1	0.3
Gross Unrealized Losses	(0.8)	(4.4)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	462.1	204.9
Debt securities issued by corporations
Investment securities
Cost or Amortized Cost	1,444.5	1,085.9
Gross Unrealized Gains	4.6	5.6
Gross Unrealized Losses	(31.4)	(45.4)
Net Foreign Currency Gains (Losses)	(3.5)	(1.1)
Carrying Value	1,414.2	1,045.0
Municipal obligations
Investment securities
Cost or Amortized Cost	3.2	275.1
Gross Unrealized Gains	0.0	0.9
Gross Unrealized Losses	0.0	(15.1)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	3.2	260.9
Mortgage and asset-backed securities
Investment securities
Cost or Amortized Cost	400.2	417.2
Gross Unrealized Gains	0.2	2.0
Gross Unrealized Losses	(26.5)	(29.8)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	373.9	389.4
Collateralized loan obligations
Investment securities
Cost or Amortized Cost	237.3	211.2
Gross Unrealized Gains	1.2	0.4
Gross Unrealized Losses	(0.2)	(2.4)
Net Foreign Currency Gains (Losses)	(1.6)	(0.1)
Carrying Value	236.7	209.1
Foreign government and agency obligations
Investment securities
Cost or Amortized Cost	22.2
Gross Unrealized Gains	0.0
Gross Unrealized Losses	(0.1)
Net Foreign Currency Gains (Losses)	(0.6)
Carrying Value	21.5
Fixed maturity investments
Investment securities
Cost or Amortized Cost	2,569.2	2,198.4
Gross Unrealized Gains	7.1	9.2
Gross Unrealized Losses	(59.0)	(97.1)
Net Foreign Currency Gains (Losses)	(5.7)	(1.2)
Carrying Value	$ 2,511.6	$ 2,109.3