Investments - Mortgage Loan Credit Monitoring (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Vintage year, Direct financing leases
2022	$ 147.1
2021	93.8	$ 94.2
2020	114.4	113.1
2019	15.3	25.3
2018	54.6	57.3
2017		32.1
Prior		287.9
Prior	239.8
Direct financing leases, Total amortized cost	665.0	609.9
Reinsurance recoverable and deposit receivable	22,032.3	1,189.2
Accrued interest receivable	578.0	542.6
Accrued interest receivable on direct financing leases	0.0	1.2
A- and above
Vintage year, Direct financing leases
2022	110.5
2021	13.0	11.7
2020	39.9	41.8
2019	1.4	1.4
2018	42.9	39.4
2017		16.6
Prior		235.6
Prior	167.3
Direct financing leases, Total amortized cost	375.0	346.5
BBB+ thru BBB-
Vintage year, Direct financing leases
2022	33.9
2021	21.9	30.2
2020	62.4	57.9
2019	11.9	22.0
2018	11.7	17.9
2017		15.5
Prior		50.2
Prior	70.4
Direct financing leases, Total amortized cost	212.2	193.7
BB+ thru BB-
Vintage year, Direct financing leases
2022	2.7
2021	57.3	50.8
2020	12.1	13.4
2019	2.0	1.9
Prior		2.1
Prior	2.1
Direct financing leases, Total amortized cost	76.2	68.2
B+ and below
Vintage year, Direct financing leases
2021	1.6	1.5
Direct financing leases, Total amortized cost	1.6	1.5
Commercial mortgage loans
Vintage year, Mortgage loans
2022	1,526.0
2021	2,734.3	2,587.3
2020	1,967.1	2,033.6
2019	2,628.9	2,706.9
2018	2,444.0	2,523.5
2017		1,739.3
Prior	5,430.0
Prior		4,804.1
Mortgage loans, Total amortized cost	16,730.3	16,394.7
Vintage year, Direct financing leases
Accrued interest receivable	57.7	60.7
Commercial mortgage loans | A- and above
Vintage year, Mortgage loans
2022	1,036.4
2021	2,277.9	2,275.9
2020	1,807.3	1,722.7
2019	2,210.2	2,412.9
2018	2,187.9	2,383.3
2017		1,437.2
Prior	4,624.9
Prior		4,334.2
Mortgage loans, Total amortized cost	14,144.6	14,566.2
Commercial mortgage loans | BBB+ thru BBB-
Vintage year, Mortgage loans
2022	385.6
2021	439.6	278.6
2020	156.8	305.6
2019	418.7	294.0
2018	238.9	131.4
2017		302.1
Prior	691.7
Prior		380.0
Mortgage loans, Total amortized cost	2,331.3	1,691.7
Commercial mortgage loans | BB+ thru BB-
Vintage year, Mortgage loans
2022	104.0
2021	16.8	32.8
2020	3.0	5.3
2018	8.9
Prior	71.7
Prior		55.4
Mortgage loans, Total amortized cost	204.4	93.5
Commercial mortgage loans | B+ and below
Vintage year, Mortgage loans
2018	8.3	8.8
Prior	41.7
Prior		34.5
Mortgage loans, Total amortized cost	50.0	43.3
Residential mortgage loans
Vintage year, Mortgage loans
2022	1,152.8
2021	1,745.4	2,039.1
2020	449.3	511.9
2019	134.7	156.2
2018	76.0	91.2
2017		103.2
Prior	424.8
Prior		418.3
Mortgage loans, Total amortized cost	3,983.0	3,319.9
Vintage year, Direct financing leases
Accrued interest receivable	19.6	16.7
Residential mortgage loans | Performing
Vintage year, Mortgage loans
2022	1,144.8
2021	1,740.7	2,039.1
2020	447.1	510.1
2019	133.0	155.6
2018	75.4	91.2
2017		102.4
Prior	417.8
Prior		415.6
Mortgage loans, Total amortized cost	3,958.8	3,314.0
Residential mortgage loans | Non-performing
Vintage year, Mortgage loans
2022	8.0
2021	4.7
2020	2.2	1.8
2019	1.7	0.6
2018	0.6
2017		0.8
Prior	7.0
Prior		2.7
Mortgage loans, Total amortized cost	$ 24.2	$ 5.9
Vintage year, Direct financing leases
Mortgage loans, Days delinquent to be considered non-performing	90 days