Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Dec. 31, 2022
Vintage year, Direct financing leases
2023	$ 44.3
2022	140.0	$ 147.1
2021	47.2	93.8
2020	108.4	114.4
2019	7.7	15.3
2018		54.6
Prior	286.3	239.8
Direct financing leases, Total amortized cost	633.9	665.0
Reinsurance recoverable and deposit receivable	20,221.8	21,156.7
Accrued interest receivable	650.0	578.0
Accrued interest receivable on direct financing leases	0.0	0.0
A- and above
Vintage year, Direct financing leases
2023	1.1
2022	117.2	110.5
2021	12.0	13.0
2020	55.2	39.9
2019	1.3	1.4
2018		42.9
Prior	210.6	167.3
Direct financing leases, Total amortized cost	397.4	375.0
BBB+ thru BBB-
Vintage year, Direct financing leases
2023	2.6
2022	22.8	33.9
2021	20.0	21.9
2020	45.5	62.4
2019	6.4	11.9
2018		11.7
Prior	74.1	70.4
Direct financing leases, Total amortized cost	171.4	212.2
BB+ thru BB-
Vintage year, Direct financing leases
2023	40.6
2022		2.7
2021	15.2	57.3
2020	7.7	12.1
2019		2.0
Prior	1.6	2.1
Direct financing leases, Total amortized cost	65.1	76.2
B+ and below
Vintage year, Direct financing leases
2021		1.6
Direct financing leases, Total amortized cost		1.6
Commercial mortgage loans
Vintage year, Mortgage loans
2023	720.6
2022	1,738.7	1,526.0
2021	2,752.5	2,734.3
2020	1,870.8	1,967.1
2019	2,416.4	2,628.9
2018		2,444.0
Prior	6,948.7	5,430.0
Mortgage loans, Total amortized cost	16,447.7	16,730.3
Vintage year, Direct financing leases
Accrued interest receivable	51.7	57.7
Commercial mortgage loans | A- and above
Vintage year, Mortgage loans
2023	412.0
2022	1,152.7	1,036.4
2021	2,327.8	2,277.9
2020	1,748.2	1,807.3
2019	2,117.0	2,210.2
2018		2,187.9
Prior	5,839.4	4,624.9
Mortgage loans, Total amortized cost	13,597.1	14,144.6
Commercial mortgage loans | BBB+ thru BBB-
Vintage year, Mortgage loans
2023	207.2
2022	452.7	385.6
2021	408.6	439.6
2020	119.7	156.8
2019	180.5	418.7
2018		238.9
Prior	876.2	691.7
Mortgage loans, Total amortized cost	2,244.9	2,331.3
Commercial mortgage loans | BB+ thru BB-
Vintage year, Mortgage loans
2023	101.4
2022	133.3	104.0
2021	16.1	16.8
2020	2.9	3.0
2019	118.9
2018		8.9
Prior	169.3	71.7
Mortgage loans, Total amortized cost	541.9	204.4
Commercial mortgage loans | B+ and below
Vintage year, Mortgage loans
2018		8.3
Prior	63.8	41.7
Mortgage loans, Total amortized cost	63.8	50.0
Residential mortgage loans
Vintage year, Mortgage loans
2023	465.3
2022	1,081.5	1,152.8
2021	1,365.6	1,745.4
2020	424.2	449.3
2019	124.1	134.7
2018		76.0
Prior	436.0	424.8
Mortgage loans, Total amortized cost	3,896.7	3,983.0
Vintage year, Direct financing leases
Accrued interest receivable	11.1	19.6
Residential mortgage loans | Performing
Vintage year, Mortgage loans
2023	464.9
2022	1,077.9	1,144.8
2021	1,361.0	1,740.7
2020	421.1	447.1
2019	122.0	133.0
2018		75.4
Prior	430.3	417.8
Mortgage loans, Total amortized cost	3,877.2	3,958.8
Residential mortgage loans | Non-performing
Vintage year, Mortgage loans
2023	0.4
2022	3.6	8.0
2021	4.6	4.7
2020	3.1	2.2
2019	2.1	1.7
2018		0.6
Prior	5.7	7.0
Mortgage loans, Total amortized cost	$ 19.5	24.2
Vintage year, Direct financing leases
Mortgage loans, delinquent to be considered non-performing (in days)	90 days
Other loans
Vintage year, Mortgage loans
2023	$ 103.4
Mortgage loans, Total amortized cost	103.4
Vintage year, Direct financing leases
Accrued interest receivable	1.3	$ 0.0
Other loans | Performing
Vintage year, Mortgage loans
2023	103.4
Mortgage loans, Total amortized cost	$ 103.4