Investments - Fair Value and Gross Unrealized Losses (Details) $ in Millions	Mar. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Fixed maturity securities, Fair Value
Number of positions with a gross unrealized loss, 12 Months or Less \| security	548	308
Number of position with a gross unrealized loss, more than 12 months \| security	200	123
Number of position with a gross unrealized loss, Total \| security	748	431
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less	13.30%	8.90%
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months	6.50%	6.70%
Fair value as a percentage of total fixed maturities and equity securities fair value, Total	19.80%	15.60%
Mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 24.0	$ 4.9
Fixed maturity securities, Fair Value, More than 12 Months	3.1	2.6
Fixed maturity securities, Fair Value, Total	27.1	7.5
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	1.5	0.1
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.2	0.2
Fixed maturity securities, Gross Unrealized Losses, Total	1.7	0.3
Other
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	172.4	95.9
Fixed maturity securities, Fair Value, More than 12 Months	0.0	0.0
Fixed maturity securities, Fair Value, Total	172.4	95.9
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	11.3	1.0
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.0	0.0
Fixed maturity securities, Gross Unrealized Losses, Total	11.3	1.0
Municipal bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	126.5	18.1
Fixed maturity securities, Fair Value, More than 12 Months	5.4	0.0
Fixed maturity securities, Fair Value, Total	131.9	18.1
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	3.4	0.5
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.1	0.0
Fixed maturity securities, Gross Unrealized Losses, Total	3.5	0.5
Foreign government bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	0.5	0.0
Fixed maturity securities, Fair Value, More than 12 Months	0.0	0.0
Fixed maturity securities, Fair Value, Total	0.5	0.0
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	382.1	126.6
Fixed maturity securities, Fair Value, More than 12 Months	47.1	10.9
Fixed maturity securities, Fair Value, Total	429.2	137.5
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	16.4	3.7
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	1.6	0.2
Fixed maturity securities, Gross Unrealized Losses, Total	18.0	3.9
Other asset-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	137.1	316.9
Fixed maturity securities, Fair Value, More than 12 Months	359.4	409.3
Fixed maturity securities, Fair Value, Total	496.5	726.2
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	1.3	17.2
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	12.8	5.5
Fixed maturity securities, Gross Unrealized Losses, Total	14.1	22.7
Total Fixed Maturity Securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	842.6	562.4
Fixed maturity securities, Fair Value, More than 12 Months	415.0	422.8
Fixed maturity securities, Fair Value, Total	1,257.6	985.2
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	33.9	22.5
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	14.7	5.9
Fixed maturity securities, Gross Unrealized Losses, Total	$ 48.6	$ 28.4

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