Investment Securities - Investment Holdings (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020
U.S. Government and agency obligations
Investment securities
Cost or Amortized Cost	$ 212.1	$ 173.2
Gross Unrealized Gains	0.5	3.1
Gross Unrealized Losses	(1.1)	0.0
Net Foreign Currency Gains (Losses)	0.0
Carrying Value	211.5	176.3
Debt securities issued by corporations
Investment securities
Cost or Amortized Cost	993.3	522.8
Gross Unrealized Gains	8.7	24.7
Gross Unrealized Losses	(8.7)	(0.1)
Net Foreign Currency Gains (Losses)	(0.4)
Carrying Value	992.9	547.4
Municipal obligations
Investment securities
Cost or Amortized Cost	276.4	244.0
Gross Unrealized Gains	16.8	21.0
Gross Unrealized Losses	(1.3)	0.0
Net Foreign Currency Gains (Losses)	0.0
Carrying Value	291.9	265.0
Mortgage and asset-backed securities
Investment securities
Cost or Amortized Cost	277.2	211.7
Gross Unrealized Gains	2.9	6.8
Gross Unrealized Losses	(2.5)	0.0
Net Foreign Currency Gains (Losses)	0.0
Carrying Value	277.6	218.5
Collateralized loan obligations
Investment securities
Cost or Amortized Cost	136.5
Gross Unrealized Gains	0.0
Gross Unrealized Losses	(0.4)
Net Foreign Currency Gains (Losses)	(1.1)
Carrying Value	135.0
Total fixed maturity investments
Investment securities
Cost or Amortized Cost	1,895.5	1,151.7
Gross Unrealized Gains	28.9	55.6
Gross Unrealized Losses	(14.0)	(0.1)
Net Foreign Currency Gains (Losses)	(1.5)
Carrying Value	$ 1,908.9	$ 1,207.2
Including short term fixed
Investment securities
Duration of fixed maturities (years)	2 years 7 months 6 days
Excluding short term investments
Investment securities
Duration of fixed maturities (years)	3 years 2 months 12 days

Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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Debt Securities, Held-to-Maturity, Net Foreign Currency Gains

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Duration of fixed maturity investments in years

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