Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	$ 7,811.8	$ 7,358.2
Gross Unrealized Gains	29.4	668.9
Gross Unrealized Losses	(936.8)	(32.7)
Fair Value	6,894.8	7,986.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	9.6	7.5	$ 3.3
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	612.5	610.1
Gross Unrealized Gains	1.3	29.2
Gross Unrealized Losses	(85.8)	(1.9)
Fair Value	528.0	637.4
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	1,797.6	1,752.5
Gross Unrealized Gains	10.3	144.6
Gross Unrealized Losses	(238.3)	(7.0)
Fair Value	1,568.9	1,890.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.7	0.0
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	5.0	6.7
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.9)	(1.2)
Fair Value	4.1	5.5
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0	0.3
Corporate Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	4,030.3	3,929.0
Gross Unrealized Gains	17.7	481.4
Gross Unrealized Losses	(499.7)	(16.0)
Fair Value	3,539.4	4,386.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	8.9	7.5	$ 3.0
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	9.0	7.0
Gross Unrealized Gains	0.0	0.4
Gross Unrealized Losses	(1.0)	0.0
Fair Value	8.0	7.4
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	1,014.7	756.0
Gross Unrealized Gains	0.0	0.9
Gross Unrealized Losses	(60.8)	(4.8)
Fair Value	953.9	752.1
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.0
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	342.7	296.9
Gross Unrealized Gains	0.1	12.4
Gross Unrealized Losses	(50.3)	(1.8)
Fair Value	292.5	307.5
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0.0	$ 0.0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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