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Investments - Mortgage Loans, Non-Accrual and Aging (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	$ 11.4	$ 11.8
Financing receivables, Non-accrual assets without a valuation allowance	0.8
Financing receivables, Nonaccrual, Interest Income	0.0
Past due	64.2	68.2
Current	17,271.1	16,445.7
Financing receivables, Total amortized cost	17,335.3	16,513.9	$ 15,625.1
Recorded investment 90 days or more past due and accruing	2.8	2.7
30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	45.7	47.4
60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	8.0	9.3
90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	10.5	11.5
Commercial mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	4.5	5.0
Past due	10.2
Current	15,132.4
Financing receivables, Total amortized cost	15,142.6	15,017.2	14,274.2
Commercial mortgage loans \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	5.8
Commercial mortgage loans \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	4.4
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	4.9
Current	640.7
Financing receivables, Total amortized cost	645.6
Direct financing leases \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	4.9
Residential mortgage loans
Financing receivables, non-accrual and aging disclosures
Financing receivables, Non-accrual status	6.9	6.8
Financing receivables, Non-accrual assets without a valuation allowance	0.8
Past due	49.1
Current	1,498.0
Financing receivables, Total amortized cost	1,547.1	1,496.7	$ 1,350.9
Recorded investment 90 days or more past due and accruing	2.8
Residential mortgage loans \| 30 to 59 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	35.0
Residential mortgage loans \| 60 to 89 Days Past Due
Financing receivables, non-accrual and aging disclosures
Past due	8.0
Residential mortgage loans \| 90 Days or More Past Due
Financing receivables, non-accrual and aging disclosures
Past due	$ 6.1
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