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Investments - Financing Receivable, Non-Accrual and Aging (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	$ 10.8	$ 21.5	$ 11.8
Financing receivables, Non-accrual assets without a valuation allowance	0.7	0.7
Financing receivables, Nonaccrual, Interest Income	0.0	0.0
Financing receivables, Total amortized cost	20,086.4	18,103.9
Recorded investment 90 days or more past due and accruing	9.3	7.8
Reinsurance recoverables	1,122.7	1,098.0
Current
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	20,020.5	17,951.1
Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	65.9	152.8
Reinsurance recoverables	0.0	0.0
30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	40.1	108.7
60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	7.5	21.2
90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	18.3	22.9
Commercial mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	4.6	10.7	5.0
Financing receivables, Total amortized cost	16,332.6	15,441.6
Recorded investment 90 days or more past due and accruing	2.7	1.3
Commercial mortgage loans \| Current
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	16,319.9	15,378.3
Commercial mortgage loans \| Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	12.7	63.3
Commercial mortgage loans \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	5.3	42.1
Commercial mortgage loans \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	0.1	9.2
Commercial mortgage loans \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	7.3	12.0
Direct financing leases
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	623.4	710.8
Recorded investment 90 days or more past due and accruing	3.6
Direct financing leases \| Current
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	619.8	707.6
Direct financing leases \| Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	3.6	3.2
Direct financing leases \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost		3.2
Direct financing leases \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	3.6
Residential mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	6.2	10.8	$ 6.8
Financing receivables, Non-accrual assets without a valuation allowance	0.7	0.7
Financing receivables, Total amortized cost	3,130.4	1,951.5
Recorded investment 90 days or more past due and accruing	3.0	6.5
Residential mortgage loans \| Current
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	3,080.8	1,865.2
Residential mortgage loans \| Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	49.6	86.3
Residential mortgage loans \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	34.8	66.6
Residential mortgage loans \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	7.4	8.8
Residential mortgage loans \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Financing receivables, Total amortized cost	$ 7.4	$ 10.9