Investments - Mortgage Loans, Non-Accrual and Aging (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	$ 11.8	$ 14.4
Past due	68.2	66.6
Current	16,445.7	15,297.7
Mortgage loans, Total amortized cost	16,513.9	15,364.3	$ 14,183.2
Recorded investment 90 days or more past due and accruing	2.7	2.5
30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	47.4	45.1
60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	9.3	9.0
90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	11.5	12.5
Commercial mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	15,017.2	13,996.3	12,897.3
Brick and mortar
Mortgage loan non-accrual and aging disclosures
Current	14,856.7	13,806.3
Mortgage loans, Total amortized cost	14,856.7	13,806.3
Credit tenant loans
Mortgage loan non-accrual and aging disclosures
Current	160.5	190.0
Mortgage loans, Total amortized cost	160.5	190.0
Residential mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	1,496.7	1,368.0	$ 1,285.9
First liens
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	8.8	10.1
Past due	67.1	64.8
Current	1,418.6	1,288.1
Mortgage loans, Total amortized cost	1,485.7	1,352.9
Recorded investment 90 days or more past due and accruing	2.7	2.5
First liens \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	46.6	44.3
First liens \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	9.3	8.4
First liens \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	11.2	12.1
Home equity
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	3.0	4.3
Past due	1.1	1.8
Current	9.9	13.3
Mortgage loans, Total amortized cost	11.0	15.1
Home equity \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	0.8	0.8
Home equity \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due		0.6
Home equity \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	$ 0.3	$ 0.4

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

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