Investment Securities - Investment Holdings Fixed Maturity Table (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Debt and Equity Securities, FV-NI [Line Items]
Net Foreign Currency Gains	$ 0.1
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	220.1	$ 173.2
Gross Unrealized Gains	2.1	3.1
Gross Unrealized Losses	(0.6)	0.0
Net Foreign Currency Gains	0.0
Carrying Value	221.6	176.3
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	678.6	522.8
Gross Unrealized Gains	13.9	24.7
Gross Unrealized Losses	(5.7)	(0.1)
Net Foreign Currency Gains	0.1
Carrying Value	686.9	547.4
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	269.4	244.0
Gross Unrealized Gains	15.4	21.0
Gross Unrealized Losses	(1.0)	0.0
Net Foreign Currency Gains	0.0
Carrying Value	283.8	265.0
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	225.8	211.7
Gross Unrealized Gains	5.4	6.8
Gross Unrealized Losses	(1.3)	0.0
Net Foreign Currency Gains	0.0
Carrying Value	229.9	218.5
Total fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,453.4	1,151.7
Gross Unrealized Gains	36.8	55.6
Gross Unrealized Losses	(8.7)	(0.1)
Carrying Value	1,481.6	$ 1,207.2
Collateralized loan obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	59.5
Gross Unrealized Gains	0.0
Gross Unrealized Losses	(0.1)
Net Foreign Currency Gains	0.0
Carrying Value	$ 59.4

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

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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
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