Investment Securities - Investment Holdings Fixed Maturity Table (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
U.S. Government and agency obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	$ 206.2	$ 231.7
Gross Unrealized Gains	4.4	1.0
Gross Unrealized Losses	0.0	(0.2)
Carrying Value	210.6	232.5
Debt securities issued by corporations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	505.4	454.9
Gross Unrealized Gains	24.0	12.5
Gross Unrealized Losses	(0.7)	(0.2)
Carrying Value	528.7	467.2
Municipal obligations
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	251.0	284.7
Gross Unrealized Gains	20.4	12.5
Gross Unrealized Losses	(0.1)	(0.1)
Carrying Value	271.3	297.1
Mortgage and asset-backed securities
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	203.6	206.6
Gross Unrealized Gains	7.3	2.7
Gross Unrealized Losses	0.0	(0.3)
Carrying Value	210.9	209.0
Total fixed maturity investments
Debt and Equity Securities, FV-NI [Line Items]
Cost or Amortized Cost	1,166.2	1,177.9
Gross Unrealized Gains	56.1	28.7
Gross Unrealized Losses	(0.8)	(0.8)
Carrying Value	$ 1,221.5	$ 1,205.8