Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	$ 7,565.8	$ 7,811.8
Gross Unrealized Gains	54.6	29.4
Gross Unrealized Losses	(730.3)	(936.8)
Fair Value	6,881.9	6,894.8
Allowance for credit loss, available for sale	8.2	9.6	$ 7.5
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	594.1	612.5
Gross Unrealized Gains	1.9	1.3
Gross Unrealized Losses	(84.5)	(85.8)
Fair Value	511.5	528.0
Allowance for credit loss, available for sale	0.0	0.0
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	1,575.9	1,797.6
Gross Unrealized Gains	16.3	10.3
Gross Unrealized Losses	(189.8)	(238.3)
Fair Value	1,401.9	1,568.9
Allowance for credit loss, available for sale	0.5	0.7
Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	4.4	5.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.6)	(0.9)
Fair Value	3.8	4.1
Allowance for credit loss, available for sale	0.0	0.0
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	4,046.8	4,030.3
Gross Unrealized Gains	35.5	17.7
Gross Unrealized Losses	(383.8)	(499.7)
Fair Value	3,690.8	3,539.4
Allowance for credit loss, available for sale	7.7	8.9	$ 7.5
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	9.0	9.0
Gross Unrealized Gains	0.1	0.0
Gross Unrealized Losses	(0.8)	(1.0)
Fair Value	8.3	8.0
Allowance for credit loss, available for sale	0.0	0.0
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	973.6	1,014.7
Gross Unrealized Gains	0.7	0.0
Gross Unrealized Losses	(24.5)	(60.8)
Fair Value	949.8	953.9
Allowance for credit loss, available for sale	0.0	0.0
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	362.0	342.7
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(46.3)	(50.3)
Fair Value	315.8	292.5
Allowance for credit loss, available for sale	$ 0.0	$ 0.0