Investments - Gross Unrealized Losses of Fixed Maturities and Equity Securities in Unrealized Loss Position (Details) $ in Millions	Dec. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Fixed maturity securities, Gross Unrealized Losses
Number of positions with a gross unrealized loss, 12 Months or Less \| security	516	308
Number of position with a gross unrealized loss, more than 12 months \| security	122	123
Number of position with a gross unrealized loss, Total \| security	638	431
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less	12.60%	8.90%
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months	3.40%	6.70%
Fair value as a percentage of total fixed maturities and equity securities fair value, Total	16.00%	15.60%
Total
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 783.4	$ 562.4
Fixed maturity securities, Fair Value, More than 12 Months	211.9	422.8
Fixed maturity securities, Fair Value, Total	995.3	985.2
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	11.3	22.5
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	7.3	5.9
Fixed maturity securities, Gross Unrealized Losses, Total	18.6	28.4
Mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	67.4	4.9
Fixed maturity securities, Fair Value, More than 12 Months	3.9	2.6
Fixed maturity securities, Fair Value, Total	71.3	7.5
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	1.3	0.1
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.2	0.2
Fixed maturity securities, Gross Unrealized Losses, Total	1.5	0.3
Other
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	59.5	95.9
Fixed maturity securities, Fair Value, More than 12 Months	35.1	0.0
Fixed maturity securities, Fair Value, Total	94.6	95.9
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	1.7	1.0
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	2.6	0.0
Fixed maturity securities, Gross Unrealized Losses, Total	4.3	1.0
Municipal bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	56.8	18.1
Fixed maturity securities, Fair Value, More than 12 Months	0.6	0.0
Fixed maturity securities, Fair Value, Total	57.4	18.1
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	0.7	0.5
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.0	0.0
Fixed maturity securities, Gross Unrealized Losses, Total	0.7	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	220.7	126.6
Fixed maturity securities, Fair Value, More than 12 Months	44.1	10.9
Fixed maturity securities, Fair Value, Total	264.8	137.5
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	3.8	3.7
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	1.4	0.2
Fixed maturity securities, Gross Unrealized Losses, Total	5.2	3.9
Other asset-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	379.0	316.9
Fixed maturity securities, Fair Value, More than 12 Months	128.2	409.3
Fixed maturity securities, Fair Value, Total	507.2	726.2
Fixed maturity securities, Gross Unrealized Losses
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	3.8	17.2
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	3.1	5.5
Fixed maturity securities, Gross Unrealized Losses, Total	$ 6.9	$ 22.7

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Debt Securities, Available-for-sale, Unrealized Loss Position Abstract [Abstract]

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