Investment Securities - Investment Holdings Fixed Maturity Table (Details) - USD ($) $ in Millions	Jun. 30, 2022	Dec. 31, 2021
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 233.0	$ 212.1
Gross Unrealized Gains	0.0	0.5
Gross Unrealized Losses	(7.2)	(1.1)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	225.8	211.5
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,043.5	993.3
Gross Unrealized Gains	0.5	8.7
Gross Unrealized Losses	(67.4)	(8.7)
Net Foreign Currency Gains (Losses)	(2.4)	(0.4)
Carrying Value	974.2	992.9
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	291.8	276.4
Gross Unrealized Gains	1.0	16.8
Gross Unrealized Losses	(12.5)	(1.3)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	280.3	291.9
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	268.5	277.2
Gross Unrealized Gains	0.0	2.9
Gross Unrealized Losses	(23.9)	(2.5)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	244.6	277.6
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	150.8	136.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(6.0)	(0.4)
Net Foreign Currency Gains (Losses)	(3.0)	(1.1)
Carrying Value	141.8	135.0
Total fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,987.6	1,895.5
Gross Unrealized Gains	1.5	28.9
Gross Unrealized Losses	(117.0)	(14.0)
Net Foreign Currency Gains (Losses)	(5.4)	(1.5)
Carrying Value	$ 1,866.7	$ 1,908.9