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Investments - Financing Receivable, Non-Accrual and Aging (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	$ 52.6	$ 17.2	$ 21.5
Financing receivables, Non-accrual assets without a valuation allowance	0.7	0.7
Financing receivables, Nonaccrual, Interest Income	0.3
Financing receivables, Total amortized cost	21,281.3	20,324.5
Recorded investment 90 days or more past due and accruing	5.2	5.3
Reinsurance recoverables	1,115.3	1,189.3
Current
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	21,134.7	20,240.1
Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	146.6	84.4
Reinsurance recoverables	0.0	0.0
30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	106.3	59.9
60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	13.3	11.5
90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	27.0	13.0
Commercial mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	46.5	13.2	10.7
Financing receivables, Nonaccrual, Interest Income	0.3
Financing receivables, Total amortized cost	16,860.2	16,394.7
Recorded investment 90 days or more past due and accruing		2.7
Commercial mortgage loans \| Current
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	16,835.5	16,386.9
Commercial mortgage loans \| Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	24.7	7.8
Commercial mortgage loans \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	7.4	0.7
Commercial mortgage loans \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	0.8
Commercial mortgage loans \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	16.5	7.1
Direct financing leases
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	670.7	609.9
Recorded investment 90 days or more past due and accruing		0.7
Direct financing leases \| Current
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	668.6	607.3
Direct financing leases \| Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	2.1	2.6
Direct financing leases \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	2.1	1.9
Direct financing leases \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost		0.7
Residential mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	6.1	4.0	$ 10.8
Financing receivables, Non-accrual assets without a valuation allowance	0.7	0.7
Financing receivables, Total amortized cost	3,750.4	3,319.9
Recorded investment 90 days or more past due and accruing	5.2	1.9
Residential mortgage loans \| Current
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	3,630.6	3,245.9
Residential mortgage loans \| Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	119.8	74.0
Residential mortgage loans \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	98.9	59.2
Residential mortgage loans \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	10.4	9.6
Residential mortgage loans \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	$ 10.5	$ 5.2