Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Dec. 31, 2022
Vintage year, Direct financing leases
2023	$ 2.2
2022	155.3	$ 147.1
2021	99.9	93.8
2020	123.6	114.4
2019	14.3	15.3
2018		54.6
Prior	321.8	239.8
Direct financing leases, Total amortized cost	717.1	665.0
Reinsurance recoverable and deposit receivable	20,884.0	21,156.7
Accrued interest receivable	632.6	578.0
Accrued interest receivable on direct financing leases	1.7	0.0
A- and above
Vintage year, Direct financing leases
2022	129.8	110.5
2021	14.7	13.0
2020	62.7	39.9
2019	1.5	1.4
2018		42.9
Prior	235.8	167.3
Direct financing leases, Total amortized cost	444.5	375.0
BBB+ thru BBB-
Vintage year, Direct financing leases
2022	25.5	33.9
2021	23.2	21.9
2020	52.2	62.4
2019	12.8	11.9
2018		11.7
Prior	84.1	70.4
Direct financing leases, Total amortized cost	197.8	212.2
BB+ thru BB-
Vintage year, Direct financing leases
2023	2.2
2022		2.7
2021	62.0	57.3
2020	8.7	12.1
2019		2.0
Prior	1.9	2.1
Direct financing leases, Total amortized cost	74.8	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	117.7
2022	1,610.0	1,526.0
2021	2,764.5	2,734.3
2020	1,968.2	1,967.1
2019	2,597.6	2,628.9
2018		2,444.0
Prior	7,609.1	5,430.0
Mortgage loans, Total amortized cost	16,667.1	16,730.3
Vintage year, Direct financing leases
Accrued interest receivable	54.3	57.7
Commercial mortgage loans | A- and above
Vintage year, Mortgage loans
2023	71.0
2022	1,039.4	1,036.4
2021	2,302.1	2,277.9
2020	1,779.6	1,807.3
2019	2,206.7	2,210.2
2018		2,187.9
Prior	6,595.5	4,624.9
Mortgage loans, Total amortized cost	13,994.3	14,144.6
Commercial mortgage loans | BBB+ thru BBB-
Vintage year, Mortgage loans
2023	14.9
2022	454.2	385.6
2021	444.2	439.6
2020	185.4	156.8
2019	390.9	418.7
2018		238.9
Prior	827.2	691.7
Mortgage loans, Total amortized cost	2,316.8	2,331.3
Commercial mortgage loans | BB+ thru BB-
Vintage year, Mortgage loans
2023	31.8
2022	116.4	104.0
2021	18.2	16.8
2020	3.2	3.0
2018		8.9
Prior	136.9	71.7
Mortgage loans, Total amortized cost	306.5	204.4
Commercial mortgage loans | B+ and below
Vintage year, Mortgage loans
2018		8.3
Prior	49.5	41.7
Mortgage loans, Total amortized cost	49.5	50.0
Residential mortgage loans
Vintage year, Mortgage loans
2023	81.6
2022	1,119.3	1,152.8
2021	1,735.1	1,745.4
2020	452.4	449.3
2019	134.3	134.7
2018		76.0
Prior	501.5	424.8
Mortgage loans, Total amortized cost	4,024.2	3,983.0
Vintage year, Direct financing leases
Accrued interest receivable	10.9	19.6
Residential mortgage loans | Performing
Vintage year, Mortgage loans
2023	81.6
2022	1,114.0	1,144.8
2021	1,730.4	1,740.7
2020	448.6	447.1
2019	132.9	133.0
2018		75.4
Prior	493.8	417.8
Mortgage loans, Total amortized cost	4,001.3	3,958.8
Residential mortgage loans | Non-performing
Vintage year, Mortgage loans
2022	5.3	8.0
2021	4.7	4.7
2020	3.8	2.2
2019	1.4	1.7
2018		0.6
Prior	7.7	7.0
Mortgage loans, Total amortized cost	$ 22.9	$ 24.2
Vintage year, Direct financing leases
Mortgage loans, delinquent to be considered non-performing (in days)	90 days