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Investments - Financing Receivable, Non-Accrual and Aging (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	$ 21.5	$ 11.8
Financing receivables, Non-accrual assets without a valuation allowance	0.7
Financing receivables, Nonaccrual, Interest Income	0.0
Past due	152.8	68.2
Current	17,951.1	16,445.7
Financing receivables, Total amortized cost	18,103.9	16,513.9	$ 15,364.3
Recorded investment 90 days or more past due and accruing	7.8	2.7
Reinsurance recoverables	0.0
30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	108.7	47.4
60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	21.2	9.3
90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	22.9	11.5
Commercial mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	10.7	5.0
Past due	63.3
Current	15,378.3
Financing receivables, Total amortized cost	15,441.6	15,017.2	13,996.3
Recorded investment 90 days or more past due and accruing	1.3
Commercial mortgage loans \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	42.1
Commercial mortgage loans \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	9.2
Commercial mortgage loans \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	12.0
Brick and mortar
Financing receivables, non-accrual and aging disclosures
Current		14,856.7
Financing receivables, Total amortized cost		14,856.7
Credit tenant loans
Financing receivables, non-accrual and aging disclosures
Current		160.5
Financing receivables, Total amortized cost		160.5
Direct financing leases
Financing receivables, non-accrual and aging disclosures
Past due	3.2
Current	707.6
Financing receivables, Total amortized cost	710.8
Direct financing leases \| 60 to 89 Days 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	10.8	6.8
Financing receivables, Non-accrual assets without a valuation allowance	0.7
Past due	86.3
Current	1,865.2
Financing receivables, Total amortized cost	1,951.5	1,496.7	$ 1,368.0
Recorded investment 90 days or more past due and accruing	6.5
Residential mortgage loans \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	66.6
Residential mortgage loans \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	8.8
Residential mortgage loans \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	$ 10.9
First liens
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status		8.8
Past due		67.1
Current		1,418.6
Financing receivables, Total amortized cost		1,485.7
Recorded investment 90 days or more past due and accruing		2.7
First liens \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due		46.6
First liens \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due		9.3
First liens \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due		11.2
Home equity
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status		3.0
Past due		1.1
Current		9.9
Financing receivables, Total amortized cost		11.0
Home equity \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due		0.8
Home equity \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due		$ 0.3