Investments - Schedule of Fixed Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	$ 7,622.5
Gross unrealized gains	9.6
Gross unrealized loss	(1,149.0)
Fixed maturities, allowance for credit losses	(8.6)	$ (9.6)	$ 7.5	$ 7.5
Fixed maturities at fair value	6,474.5
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost		7,811.8
Gross unrealized gains		29.4
Gross unrealized loss		(936.8)
Fixed maturities at fair value		6,894.8
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1,149.0	936.8
Investments in Fixed Maturities | Investment-grade
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1,120.1	904.0
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	599.7	612.5
Gross unrealized gains	0.2	1.3
Gross unrealized loss	(112.7)	(85.8)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	487.2	528.0
Debt securities, available-for-sale, unrealized loss position, accumulated loss	112.7	85.8
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	1,613.9	1,797.6
Gross unrealized gains	1.5	10.3
Gross unrealized loss	(294.7)	(238.3)
Fixed maturities, allowance for credit losses	(0.5)	(0.7)
Fixed maturities at fair value	1,320.2	1,568.9
Debt securities, available-for-sale, unrealized loss position, accumulated loss	294.7	238.3
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	5.1	5.0
Gross unrealized gains	0.0	0.0
Gross unrealized loss	(0.9)	(0.9)
Fixed maturities, allowance for credit losses	0.0	0.0	0.4	0.0
Fixed maturities at fair value	4.2	4.1
Debt securities, available-for-sale, unrealized loss position, accumulated loss	0.9	0.9
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	4,047.3	4,030.3
Gross unrealized gains	6.8	17.7
Gross unrealized loss	(651.6)	(499.7)
Fixed maturities, allowance for credit losses	(8.1)	(8.9)	7.1	$ 7.5
Fixed maturities at fair value	3,394.4	3,539.4
Debt securities, available-for-sale, unrealized loss position, accumulated loss	651.6	499.7
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	9.0	9.0
Gross unrealized gains	0.1	0.0
Gross unrealized loss	(1.3)	(1.0)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	7.8	8.0
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1.3		$ 1.0
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	985.4	1,014.7
Gross unrealized gains	0.9	0.0
Gross unrealized loss	(30.1)	(60.8)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	956.2	953.9
Debt securities, available-for-sale, unrealized loss position, accumulated loss	30.1	60.8
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	362.1	342.7
Gross unrealized gains	0.1	0.1
Gross unrealized loss	(57.7)	(50.3)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	304.5	292.5
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 57.7	$ 50.3