Investment Securities - Fixed Maturity Investments (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2024	Dec. 31, 2023
Fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 2,441.0	$ 2,198.4
Gross Unrealized Gains	4.3	9.2
Gross Unrealized Losses	(97.4)	(97.1)
Net Foreign Currency Gains (Losses)	(3.1)	(1.2)
Carrying Value	2,344.8	2,109.3
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	250.7	209.0
Gross Unrealized Gains	0.0	0.3
Gross Unrealized Losses	(1.8)	(4.4)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	248.9	204.9
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,145.8	1,085.9
Gross Unrealized Gains	2.2	5.6
Gross Unrealized Losses	(42.4)	(45.4)
Net Foreign Currency Gains (Losses)	(1.1)	(1.1)
Carrying Value	1,104.5	1,045.0
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	252.4	275.1
Gross Unrealized Gains	0.4	0.9
Gross Unrealized Losses	(15.0)	(15.1)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	237.8	260.9
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	520.7	417.2
Gross Unrealized Gains	0.5	2.0
Gross Unrealized Losses	(37.6)	(29.8)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	483.6	389.4
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	271.4	211.2
Gross Unrealized Gains	1.2	0.4
Gross Unrealized Losses	(0.6)	(2.4)
Net Foreign Currency Gains (Losses)	(2.0)	(0.1)
Carrying Value	$ 270.0	$ 209.1