Investments - Schedule of Fixed Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	$ 7,696.4
Gross unrealized gains	21.5
Gross unrealized loss	(1,031.3)
Fixed maturities, allowance for credit losses	(7.5)	$ (7.5)	$ 4.4	$ 3.3
Fixed maturities at fair value	6,679.1
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost		7,358.2
Gross unrealized gains		668.9
Gross unrealized loss		(32.7)
Fixed maturities at fair value		7,986.9
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1,031.3	32.7
Investments in Fixed Maturities | Investment-grade
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	998.2	23.7
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	612.7	610.1
Gross unrealized gains	1.3	29.2
Gross unrealized loss	(87.2)	(1.9)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	526.8	637.4
Debt securities, available-for-sale, unrealized loss position, accumulated loss	87.2	1.9
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	1,810.0	1,752.5
Gross unrealized gains	8.9	144.6
Gross unrealized loss	(266.4)	(7.0)
Fixed maturities, allowance for credit losses	(0.4)	0.0
Fixed maturities at fair value	1,552.1	1,890.1
Debt securities, available-for-sale, unrealized loss position, accumulated loss	266.4	7.0
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	4.8	6.7
Gross unrealized gains	0.0	0.0
Gross unrealized loss	(1.1)	(1.2)
Fixed maturities, allowance for credit losses	0.0	0.0	0.0	0.3
Fixed maturities at fair value	3.7	5.5
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1.1	1.2
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	3,913.9	3,929.0
Gross unrealized gains	10.5	481.4
Gross unrealized loss	(575.8)	(16.0)
Fixed maturities, allowance for credit losses	(7.1)	(7.5)	4.4	$ 3.0
Fixed maturities at fair value	3,341.5	4,386.9
Debt securities, available-for-sale, unrealized loss position, accumulated loss	575.8	16.0
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	9.0	7.0
Gross unrealized gains	0.0	0.4
Gross unrealized loss	(1.0)	0.0
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	8.0	7.4
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1.0		$ 0.0
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	1,006.9	756.0
Gross unrealized gains	0.0	0.9
Gross unrealized loss	(54.3)	(4.8)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	952.6	752.1
Debt securities, available-for-sale, unrealized loss position, accumulated loss	54.3	4.8
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	339.1	296.9
Gross unrealized gains	0.8	12.4
Gross unrealized loss	(45.5)	(1.8)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	294.4	307.5
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 45.5	$ 1.8