Investments - Financing Receivable, Non-Accrual and Aging (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	$ 10.9	$ 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
Past due	143.0	152.8
Current	18,800.2	17,951.1
Financing receivables, Total amortized cost	18,943.2	18,103.9
Recorded investment 90 days or more past due and accruing	9.6	7.8
Reinsurance recoverables	0.0	0.0
30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	117.0	108.7
60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	7.7	21.2
90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	18.3	22.9
Commercial mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	5.2	10.7	5.0
Past due	17.2	63.3
Current	15,824.7	15,378.3
Financing receivables, Total amortized cost	15,841.9	15,441.6
Recorded investment 90 days or more past due and accruing	3.0	1.3
Commercial mortgage loans \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	8.2	42.1
Commercial mortgage loans \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	0.8	9.2
Commercial mortgage loans \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	8.2	12.0
Direct financing leases
Financing receivables, non-accrual and aging disclosures
Past due	67.0	3.2
Current	652.0	707.6
Financing receivables, Total amortized cost	719.0	710.8
Recorded investment 90 days or more past due and accruing	3.2
Direct financing leases \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	62.8
Direct financing leases \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	1.0	3.2
Direct financing leases \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	3.2
Residential mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	5.7	10.8	$ 6.8
Financing receivables, Non-accrual assets without a valuation allowance	0.7	0.7
Past due	58.8	86.3
Current	2,323.5	1,865.2
Financing receivables, Total amortized cost	2,382.3	1,951.5
Recorded investment 90 days or more past due and accruing	3.4	6.5
Residential mortgage loans \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	46.0	66.6
Residential mortgage loans \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	5.9	8.8
Residential mortgage loans \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	$ 6.9	$ 10.9

Amount of interest income on financing receivable on nonaccrual status.

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Amount of financing receivable on nonaccrual status with no allowance for credit loss.

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Amount of financing receivable 90 days or more past due and still accruing.

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Amount of financing receivable past due.

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Amount, before allowance for credit loss, of financing receivable.

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Amount of reinsurance recoverable past due.

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