Investment Securities - Investment Holdings Fixed Maturity Table (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 181.4	$ 212.1
Gross Unrealized Gains	0.0	0.5
Gross Unrealized Losses	(5.3)	(1.1)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	176.1	211.5
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,013.4	993.3
Gross Unrealized Gains	1.4	8.7
Gross Unrealized Losses	(45.4)	(8.7)
Net Foreign Currency Gains (Losses)	0.2	(0.4)
Carrying Value	969.6	992.9
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	272.2	276.4
Gross Unrealized Gains	6.0	16.8
Gross Unrealized Losses	(5.9)	(1.3)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	272.3	291.9
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	278.2	277.2
Gross Unrealized Gains	0.2	2.9
Gross Unrealized Losses	(14.3)	(2.5)
Net Foreign Currency Gains (Losses)	0.0	0.0
Carrying Value	264.1	277.6
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	136.5	136.5
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(1.6)	(0.4)
Net Foreign Currency Gains (Losses)	(1.5)	(1.1)
Carrying Value	133.4	135.0
Total fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,881.7	1,895.5
Gross Unrealized Gains	7.6	28.9
Gross Unrealized Losses	(72.5)	(14.0)
Net Foreign Currency Gains (Losses)	(1.3)	(1.5)
Carrying Value	$ 1,815.5	$ 1,908.9