Investment Securities - Investment Holdings Fixed Maturity Table (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 207.4	$ 173.2
Gross Unrealized Gains	0.8	3.1
Gross Unrealized Losses	(0.6)	0.0
Net Foreign Currency Gains (Losses)	0.0
Carrying Value	207.6	176.3
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	945.2	522.8
Gross Unrealized Gains	13.1	24.7
Gross Unrealized Losses	(3.4)	(0.1)
Net Foreign Currency Gains (Losses)	(0.7)
Carrying Value	954.2	547.4
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	276.1	244.0
Gross Unrealized Gains	18.0	21.0
Gross Unrealized Losses	(0.7)	0.0
Net Foreign Currency Gains (Losses)	0.0
Carrying Value	293.4	265.0
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	247.3	211.7
Gross Unrealized Gains	4.2	6.8
Gross Unrealized Losses	(1.4)	0.0
Net Foreign Currency Gains (Losses)	0.0
Carrying Value	250.1	218.5
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	128.5
Gross Unrealized Gains	0.0
Gross Unrealized Losses	(0.4)
Net Foreign Currency Gains (Losses)	(0.5)
Carrying Value	127.6
Total fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,804.5	1,151.7
Gross Unrealized Gains	36.1	55.6
Gross Unrealized Losses	(6.5)	(0.1)
Net Foreign Currency Gains (Losses)	(1.2)
Carrying Value	$ 1,832.9	$ 1,207.2