Investment Securities - Fixed Maturity Investments (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 2,351.0	$ 2,198.4
Gross Unrealized Gains	5.0	9.2
Gross Unrealized Losses	(97.6)	(97.1)
Net Foreign Currency Gains (Losses)	(3.8)	(1.2)
Carrying Value	2,254.6	2,109.3
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	247.6	209.0
Gross Unrealized Gains	0.1	0.3
Gross Unrealized Losses	(1.8)	(4.4)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	245.9	204.9
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,118.3	1,085.9
Gross Unrealized Gains	3.0	5.6
Gross Unrealized Losses	(46.0)	(45.4)
Net Foreign Currency Gains (Losses)	(2.2)	(1.1)
Carrying Value	1,073.1	1,045.0
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	251.4	275.1
Gross Unrealized Gains	0.6	0.9
Gross Unrealized Losses	(13.0)	(15.1)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	239.0	260.9
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	484.8	417.2
Gross Unrealized Gains	0.6	2.0
Gross Unrealized Losses	(35.3)	(29.8)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	450.1	389.4
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	248.9	211.2
Gross Unrealized Gains	0.7	0.4
Gross Unrealized Losses	(1.5)	(2.4)
Net Foreign Currency Gains (Losses)	(1.6)	(0.1)
Carrying Value	$ 246.5	$ 209.1