Investments - Schedule of Fixed Maturities (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	$ 7,562.1
Gross unrealized gains	25.3
Gross unrealized loss	(808.1)
Fixed maturities, allowance for credit losses	(7.7)	$ (8.2)	$ 7.4	$ 9.6
Fixed maturities at fair value	6,771.6
Investments in Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost		7,565.8
Gross unrealized gains		54.6
Gross unrealized loss		(730.3)
Fixed maturities at fair value		6,881.9
Debt securities, available-for-sale, unrealized loss position, accumulated loss	808.1	730.3
Investments in Fixed Maturities | External Credit Rating, Investment Grade [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, unrealized loss position, accumulated loss	788.7	704.8
U.S. Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	582.3	594.1
Gross unrealized gains	1.0	1.9
Gross unrealized loss	(94.9)	(84.5)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	488.4	511.5
Debt securities, available-for-sale, unrealized loss position, accumulated loss	94.9	84.5
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	1,552.3	1,575.9
Gross unrealized gains	9.9	16.3
Gross unrealized loss	(205.8)	(189.8)
Fixed maturities, allowance for credit losses	(0.5)	(0.5)
Fixed maturities at fair value	1,355.9	1,401.9
Debt securities, available-for-sale, unrealized loss position, accumulated loss	205.8	189.8
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	5.2	4.4
Gross unrealized gains	0.0	0.0
Gross unrealized loss	(0.6)	(0.6)
Fixed maturities, allowance for credit losses	0.0	0.0	0.8	0.7
Fixed maturities at fair value	4.6	3.8
Debt securities, available-for-sale, unrealized loss position, accumulated loss	0.6	0.6
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	4,077.8	4,046.8
Gross unrealized gains	13.0	35.5
Gross unrealized loss	(448.0)	(383.8)
Fixed maturities, allowance for credit losses	(7.2)	(7.7)	6.6	$ 8.9
Fixed maturities at fair value	3,635.6	3,690.8
Debt securities, available-for-sale, unrealized loss position, accumulated loss	448.0	383.8
Mandatorily Redeemable Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	9.0	9.0
Gross unrealized gains	0.1	0.1
Gross unrealized loss	(1.1)	(0.8)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	8.0	8.3
Debt securities, available-for-sale, unrealized loss position, accumulated loss	1.1		$ 0.8
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	952.5	973.6
Gross unrealized gains	1.1	0.7
Gross unrealized loss	(15.4)	(24.5)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	938.2	949.8
Debt securities, available-for-sale, unrealized loss position, accumulated loss	15.4	24.5
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Fixed maturities, at amortized cost	383.0	362.0
Gross unrealized gains	0.2	0.1
Gross unrealized loss	(42.3)	(46.3)
Fixed maturities, allowance for credit losses	0.0	0.0
Fixed maturities at fair value	340.9	315.8
Debt securities, available-for-sale, unrealized loss position, accumulated loss	$ 42.3	$ 46.3