Investments - Fair Value and Gross Unrealized Losses (Details) $ in Millions	Jun. 30, 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	267	308
Number of position with a gross unrealized loss, more than 12 months \| security	154	123
Number of position with a gross unrealized loss, Total \| security	421	431
Fair value as a percentage of total fixed maturities and equity securities fair value, 12 Months or Less	6.50%	8.90%
Fair value as a percentage of total fixed maturities and equity securities fair value, more than 12 months	4.70%	6.70%
Fair value as a percentage of total fixed maturities and equity securities fair value, Total	11.20%	15.60%
Mortgage-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	$ 13.9	$ 4.9
Fixed maturity securities, Fair Value, More than 12 Months	3.0	2.6
Fixed maturity securities, Fair Value, Total	16.9	7.5
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	0.3	0.1
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.2	0.2
Fixed maturity securities, Gross Unrealized Losses, Total	0.5	0.3
Other
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	113.2	95.9
Fixed maturity securities, Fair Value, More than 12 Months	0.0	0.0
Fixed maturity securities, Fair Value, Total	113.2	95.9
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	5.3	1.0
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.0	0.0
Fixed maturity securities, Gross Unrealized Losses, Total	5.3	1.0
Municipal bonds
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	24.0	18.1
Fixed maturity securities, Fair Value, More than 12 Months	0.7	0.0
Fixed maturity securities, Fair Value, Total	24.7	18.1
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	0.3	0.5
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	0.1	0.0
Fixed maturity securities, Gross Unrealized Losses, Total	0.4	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, 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	142.3	126.6
Fixed maturity securities, Fair Value, More than 12 Months	38.8	10.9
Fixed maturity securities, Fair Value, Total	181.1	137.5
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	3.8	3.7
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	1.2	0.2
Fixed maturity securities, Gross Unrealized Losses, Total	5.0	3.9
Other asset-backed securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	136.0	316.9
Fixed maturity securities, Fair Value, More than 12 Months	262.8	409.3
Fixed maturity securities, Fair Value, Total	398.8	726.2
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	1.0	17.2
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	11.5	5.5
Fixed maturity securities, Gross Unrealized Losses, Total	12.5	22.7
Total Fixed Maturity Securities
Fixed maturity securities, Fair Value
Fixed maturity securities, Fair Value, 12 Months or Less	429.4	562.4
Fixed maturity securities, Fair Value, More than 12 Months	305.3	422.8
Fixed maturity securities, Fair Value, Total	734.7	985.2
Fixed maturity securities, Gross Unrealized Losses, 12 Months or Less	10.7	22.5
Fixed maturity securities, Gross Unrealized Losses, More than 12 Months	13.0	5.9
Fixed maturity securities, Gross Unrealized Losses, Total	$ 23.7	$ 28.4

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