Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	$ 7,358.2	$ 6,692.7
Gross Unrealized Gains	668.9	948.2
Gross Unrealized Losses	(32.7)	(31.7)
Fair Value	7,986.9	7,605.9
Debt Securities, Available-for-sale, Allowance for Credit Loss	7.5	3.3	$ 0.0
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	610.1	536.5
Gross Unrealized Gains	29.2	48.9
Gross Unrealized Losses	(1.9)	(0.1)
Fair Value	637.4	585.3
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	1,752.5	1,404.3
Gross Unrealized Gains	144.6	185.4
Gross Unrealized Losses	(7.0)	(0.2)
Fair Value	1,890.1	1,589.5
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	6.7	6.6
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(1.2)	(1.1)
Fair Value	5.5	5.2
Debt Securities, Available-for-sale, Allowance for Credit Loss	0.0	0.3	0.0
Corporate Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	3,929.0	3,749.5
Gross Unrealized Gains	481.4	689.5
Gross Unrealized Losses	(16.0)	(10.6)
Fair Value	4,386.9	4,425.4
Debt Securities, Available-for-sale, Allowance for Credit Loss	7.5	3.0	$ 0.0
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	7.0	7.0
Gross Unrealized Gains	0.4	0.5
Gross Unrealized Losses	0.0	0.0
Fair Value	7.4	7.5
Debt Securities, Available-for-sale, Allowance for Credit Loss
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	756.0	785.1
Gross Unrealized Gains	0.9	2.3
Gross Unrealized Losses	(4.8)	(19.7)
Fair Value	752.1	767.7
Debt Securities, Available-for-sale, Allowance for Credit Loss		0.0
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	296.9	203.7
Gross Unrealized Gains	12.4	21.6
Gross Unrealized Losses	(1.8)	0.0
Fair Value	$ 307.5	225.3
Debt Securities, Available-for-sale, Allowance for Credit Loss		$ 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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