Investments - Fair Value and Gross Unrealized Losses (Details) $ in Millions	Sep. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Fixed maturity securities, Number of Positions
Number of positions with a gross unrealized loss, 12 Months or Less \| security	401	308
Number of position with a gross unrealized loss, more than 12 months \| security	129	123
Number of position with a gross unrealized loss, Total \| security	530	431
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less	9.70%	8.90%
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months	4.00%	6.70%
Fair value as a percentage of total fixed maturities and equity securities fair value, Total	13.70%	15.60%
Mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 69.5	$ 4.9
Fixed maturity securities, Fair Value, More than 12 Months	2.1	2.6
Fixed maturity securities, Fair Value, Total	71.6	7.5
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	0.9	0.1
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.1	0.2
Fixed maturity securities, Gross Unrealized Losses, Total	1.0	0.3
Other
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	106.1	95.9
Fixed maturity securities, Fair Value, More than 12 Months	15.0	0.0
Fixed maturity securities, Fair Value, Total	121.1	95.9
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	4.4	1.0
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	1.2	0.0
Fixed maturity securities, Gross Unrealized Losses, Total	5.6	1.0
Municipal bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	52.2	18.1
Fixed maturity securities, Fair Value, More than 12 Months	1.4	0.0
Fixed maturity securities, Fair Value, Total	53.6	18.1
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	1.0	0.5
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.1	0.0
Fixed maturity securities, Gross Unrealized Losses, Total	1.1	0.5
Foreign government bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	0.0	0.0
Fixed maturity securities, Fair Value, More than 12 Months	0.0	0.0
Fixed maturity securities, Fair Value, Total	0.0	0.0
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	0.0	0.0
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.0	0.0
Fixed maturity securities, Gross Unrealized Losses, Total	0.0	0.0
Corporate bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	214.1	126.6
Fixed maturity securities, Fair Value, More than 12 Months	36.1	10.9
Fixed maturity securities, Fair Value, Total	250.2	137.5
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	5.3	3.7
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	1.3	0.2
Fixed maturity securities, Gross Unrealized Losses, Total	6.6	3.9
Other asset-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	187.6	316.9
Fixed maturity securities, Fair Value, More than 12 Months	205.4	409.3
Fixed maturity securities, Fair Value, Total	393.0	726.2
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	1.1	17.2
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	3.7	5.5
Fixed maturity securities, Gross Unrealized Losses, Total	4.8	22.7
Total Fixed Maturity Securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	629.5	562.4
Fixed maturity securities, Fair Value, More than 12 Months	260.0	422.8
Fixed maturity securities, Fair Value, Total	889.5	985.2
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	12.7	22.5
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	6.4	5.9
Fixed maturity securities, Gross Unrealized Losses, Total	$ 19.1	$ 28.4

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