Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Dec. 31, 2023
Vintage year, Direct financing leases
2024	$ 37.9
2023	3.5	$ 46.1
2022	125.5	145.5
2021	43.4	48.8
2020	94.5	111.6
2019		12.9
Prior		285.2
Prior	262.6
Direct financing leases, Total amortized cost	567.4	650.1
Reinsurance recoverable and deposit receivable	20,101.5	20,614.9
Accrued interest receivable	658.5	619.2
Accrued interest receivable on direct financing leases	0.0	1.2
A- and above
Vintage year, Direct financing leases
2023	1.0	1.1
2022	39.2	122.0
2021	11.0	12.4
2020	34.2	56.7
2019		1.3
Prior		207.3
Prior	186.2
Direct financing leases, Total amortized cost	271.6	400.8
BBB+ thru BBB-
Vintage year, Direct financing leases
2023	2.5	2.8
2022	85.7	23.5
2021	18.6	20.8
2020	53.2	46.9
2019		11.6
Prior		76.3
Prior	67.9
Direct financing leases, Total amortized cost	227.9	181.9
BB+ thru BB-
Vintage year, Direct financing leases
2024	37.9
2023		42.2
2022	0.6
2021	6.9	15.6
2020	3.5	8.0
Prior		1.6
Prior	8.5
Direct financing leases, Total amortized cost	57.4	67.4
B+ and below
Vintage year, Mortgage loans
2021	6.9
2020	3.6
Mortgage loans, Total amortized cost	10.5
Commercial mortgage loans
Vintage year, Mortgage loans
2024	450.5
2023	1,145.2	1,057.7
2022	1,777.3	1,775.3
2021	2,637.6	2,755.5
2020	1,788.4	1,802.0
2019		2,410.3
Prior	9,040.3
Prior		6,781.0
Mortgage loans, Total amortized cost	16,839.3	16,581.8
Vintage year, Direct financing leases
Accrued interest receivable	61.9	58.3
Commercial mortgage loans | A- and above
Vintage year, Mortgage loans
2024	386.8
2023	745.3	664.8
2022	1,303.4	1,277.1
2021	2,314.8	2,364.0
2020	1,574.9	1,587.3
2019		1,954.4
Prior	7,344.4
Prior		5,576.6
Mortgage loans, Total amortized cost	13,669.6	13,424.2
Commercial mortgage loans | BBB+ thru BBB-
Vintage year, Mortgage loans
2024	61.4
2023	279.1	282.4
2022	327.6	356.0
2021	308.0	374.8
2020	211.0	212.0
2019		337.3
Prior	1,211.1
Prior		911.7
Mortgage loans, Total amortized cost	2,398.2	2,474.2
Commercial mortgage loans | BB+ thru BB-
Vintage year, Mortgage loans
2024	2.3
2023	120.8	110.5
2022	146.3	142.2
2021	14.8	16.7
2020		2.7
2019		35.2
Prior	271.6
Prior		234.2
Mortgage loans, Total amortized cost	555.8	541.5
Commercial mortgage loans | B+ and below
Vintage year, Mortgage loans
2020	2.5
2019		83.4
Prior	213.2
Prior		58.5
Mortgage loans, Total amortized cost	215.7	141.9
Residential mortgage loans
Vintage year, Mortgage loans
2024	55.1
2023	464.3	503.4
2022	1,046.3	1,068.3
2021	1,316.4	1,346.7
2020	218.8	229.7
2019		122.8
Prior	507.6
Prior		425.6
Mortgage loans, Total amortized cost	3,608.5	3,696.5
Vintage year, Direct financing leases
Accrued interest receivable	11.2	11.4
Residential mortgage loans | Performing
Vintage year, Mortgage loans
2024	55.1
2023	462.3	502.3
2022	1,042.2	1,064.3
2021	1,311.9	1,341.8
2020	216.5	228.4
2019		120.8
Prior	500.9
Prior		421.0
Mortgage loans, Total amortized cost	3,588.9	3,678.6
Residential mortgage loans | Non-performing
Vintage year, Mortgage loans
2023	2.0	1.1
2022	4.1	4.0
2021	4.5	4.9
2020	2.3	1.3
2019		2.0
Prior	6.7
Prior		4.6
Mortgage loans, Total amortized cost	$ 19.6	17.9
Vintage year, Direct financing leases
Mortgage loans, delinquent to be considered non-performing (in days)	90 days
Other loans
Vintage year, Mortgage loans
2024	$ 41.8
2023	130.7	168.5
Prior	0.1
Prior		0.1
Mortgage loans, Total amortized cost	172.6	168.6
Vintage year, Direct financing leases
Accrued interest receivable	1.6	1.6
Other loans | Performing
Vintage year, Mortgage loans
2024	41.8
2023	130.7	168.5
Mortgage loans, Total amortized cost	172.5	168.5
Other loans | Non-performing
Vintage year, Mortgage loans
Prior	0.1
Prior		0.1
Mortgage loans, Total amortized cost	$ 0.1	$ 0.1