Schedule I Summary of Investments-Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2021 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	$ 7,005.0
Amount Shown in Balance Sheet	7,449.5
Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	5,797.7
Fair Value	6,239.3
Amount Shown in Balance Sheet	6,239.3
Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	147.2
Fair Value	147.2
Amount Shown in Balance Sheet	147.2
U.S. Government and federally sponsored agency obligations | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	776.4
Fair Value	828.4
Amount Shown in Balance Sheet	828.4
States, municipalities and political subdivisions | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	1,519.7
Fair Value	1,703.4
Amount Shown in Balance Sheet	1,703.4
Foreign government bonds | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	40.2
Fair Value	43.6
Amount Shown in Balance Sheet	43.6
Public utilities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	91.8
Fair Value	100.4
Amount Shown in Balance Sheet	100.4
All other corporate bonds | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	2,091.6
Fair Value	2,251.0
Amount Shown in Balance Sheet	2,251.0
Asset-backed securities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	935.4
Fair Value	943.2
Amount Shown in Balance Sheet	943.2
Residential mortgage-backed securities (non-agency) | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	21.5
Fair Value	21.6
Amount Shown in Balance Sheet	21.6
Commercial mortgage-backed securities | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	286.7
Fair Value	310.3
Amount Shown in Balance Sheet	310.3
Redeemable preferred stocks | Fixed maturity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	34.4
Fair Value	37.4
Amount Shown in Balance Sheet	37.4
Industrial, miscellaneous and all other | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	4.7
Fair Value	4.7
Amount Shown in Balance Sheet	4.7
Banking & finance and insurance companies | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	1.4
Fair Value	1.4
Amount Shown in Balance Sheet	1.4
Public utilities | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	0.2
Fair Value	0.2
Amount Shown in Balance Sheet	0.2
Non-redeemable preferred stocks | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	119.4
Fair Value	119.4
Amount Shown in Balance Sheet	119.4
Closed-end fund | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	21.5
Fair Value	21.5
Amount Shown in Balance Sheet	21.5
Limited partnership interests
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	712.8
Amount Shown in Balance Sheet	712.8
Short-term Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	157.8
Amount Shown in Balance Sheet	157.8
Policy loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	142.1
Amount Shown in Balance Sheet	142.1
Derivatives
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	7.8
Fair Value	10.7
Amount Shown in Balance Sheet	10.7
Mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	6.7
Amount Shown in Balance Sheet	6.7
Other
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties
Cost	32.9
Amount Shown in Balance Sheet	$ 32.9