Investment Securities - Investment Holdings (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
U.S. Government and agency obligations
Investment securities
Cost or Amortized Cost	$ 216.6	$ 212.1
Gross Unrealized Gains	0.0	0.5
Gross Unrealized Losses	(10.2)	(1.1)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	206.4	211.5
Debt securities issued by corporations
Investment securities
Cost or Amortized Cost	1,098.3	993.3
Gross Unrealized Gains	0.6	8.7
Gross Unrealized Losses	(78.3)	(8.7)
Net Foreign Currency Gains (Losses)	(1.8)	(0.4)
Carrying Value	1,018.8	992.9
Municipal obligations
Investment securities
Cost or Amortized Cost	281.6	276.4
Gross Unrealized Gains	0.4	16.8
Gross Unrealized Losses	(23.4)	(1.3)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	258.6	291.9
Mortgage and asset-backed securities
Investment securities
Cost or Amortized Cost	288.7	277.2
Gross Unrealized Gains	0.0	2.9
Gross Unrealized Losses	(34.5)	(2.5)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	254.2	277.6
Collateralized loan obligations
Investment securities
Cost or Amortized Cost	190.8	136.5
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(6.0)	(0.4)
Net Foreign Currency Gains (Losses)	(2.0)	(1.1)
Carrying Value	182.9	135.0
Total fixed maturity investments
Investment securities
Cost or Amortized Cost	2,076.0	1,895.5
Gross Unrealized Gains	1.1	28.9
Gross Unrealized Losses	(152.4)	(14.0)
Net Foreign Currency Gains (Losses)	(3.8)	(1.5)
Carrying Value	$ 1,920.9	$ 1,908.9
Including short term fixed
Investment securities
Duration of fixed maturities (years)	2 years 3 months 18 days
Excluding short term investments
Investment securities
Duration of fixed maturities (years)	3 years 4 months 24 days