Investment Securities - Investment Holdings (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
U.S. Government and agency obligations
Investment securities
Cost or Amortized Cost	$ 173.2	$ 231.7
Gross Unrealized Gains	3.1	1.0
Gross Unrealized Losses	0.0	(0.2)
Carrying Value	176.3	232.5
Debt securities issued by corporations
Investment securities
Cost or Amortized Cost	522.8	454.9
Gross Unrealized Gains	24.7	12.5
Gross Unrealized Losses	(0.1)	(0.2)
Carrying Value	547.4	467.2
Municipal obligations
Investment securities
Cost or Amortized Cost	244.0	284.7
Gross Unrealized Gains	21.0	12.5
Gross Unrealized Losses	0.0	(0.1)
Carrying Value	265.0	297.1
Mortgage and asset-backed securities
Investment securities
Cost or Amortized Cost	211.7	206.6
Gross Unrealized Gains	6.8	2.7
Gross Unrealized Losses	0.0	(0.3)
Carrying Value	218.5	209.0
Total fixed maturity investments
Investment securities
Cost or Amortized Cost	1,151.7	1,177.9
Gross Unrealized Gains	55.6	28.7
Gross Unrealized Losses	(0.1)	(0.8)
Carrying Value	$ 1,207.2	$ 1,205.8
Including short term fixed
Investment securities
Duration of fixed maturities (years)	3 years 2 months 12 days
Excluding short term investments
Investment securities
Duration of fixed maturities (years)	3 years 7 months 6 days

Amount of accumulated unrecognized gain 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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Duration of fixed maturity investments in years

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