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Investments - Mortgage Loans, Non-Accrual and Aging (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	$ 12.6	$ 14.4
Past due	60.2	66.6
Current	16,104.7	15,297.7
Mortgage loans, Total amortized cost	16,164.9	15,364.3	$ 14,398.0
Recorded investment 90 days or more past due and accruing	2.0	2.5
30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	42.9	45.1
60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	7.3	9.0
90 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	10.0	12.5
Commercial mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	14,830.6	13,996.3	13,071.9
Brick and mortar
Mortgage loan non-accrual and aging disclosures
Current	14,660.1	13,806.3
Mortgage loans, Total amortized cost	14,660.1	13,806.3
Credit tenant loans
Mortgage loan non-accrual and aging disclosures
Current	170.5	190.0
Mortgage loans, Total amortized cost	170.5	190.0
Residential mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	1,334.3	1,368.0	$ 1,326.1
First liens
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	9.2	10.1
Past due	59.5	64.8
Current	1,262.1	1,288.1
Mortgage loans, Total amortized cost	1,321.6	1,352.9
Recorded investment 90 days or more past due and accruing	2.0	2.5
First liens \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	42.2	44.3
First liens \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	7.3	8.4
First liens \| 90 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	10.0	12.1
Home equity
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	3.4	4.3
Past due	0.7	1.8
Current	12.0	13.3
Mortgage loans, Total amortized cost	12.7	15.1
Home equity \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	$ 0.7	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 Past Due
Mortgage loan non-accrual and aging disclosures
Past due		$ 0.4