Investments - Mortgage Loans, Non-Accrual and Aging (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	$ 10.4	$ 14.4
Past due	59.1	65.8
Current	14,124.1	13,209.3
Mortgage loans, Total amortized cost	14,183.2	13,275.1	$ 12,391.0
Recorded investment 90 days or more past due and accruing	7.6	5.6
30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	39.1	42.0
60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	8.6	12.4
90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	11.4	11.4
Commercial mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	12,897.3	12,055.2	11,265.3
Brick and mortar
Mortgage loan non-accrual and aging disclosures
Current	12,665.6	11,795.2
Mortgage loans, Total amortized cost	12,665.6	11,795.2
Credit tenant loans
Mortgage loan non-accrual and aging disclosures
Current	231.7	260.0
Mortgage loans, Total amortized cost	231.7	260.0
Residential mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	1,285.9	1,219.9	$ 1,125.7
Home equity
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	6.5	8.8
Past due	3.4	4.4
Current	19.6	161.2
Mortgage loans, Total amortized cost	23.0	165.6
Home equity \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	1.9	1.9
Home equity \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	0.7	1.1
Home equity \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	0.8	1.4
First liens
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	3.9	5.6
Past due	55.7	61.4
Current	1,207.2	992.9
Mortgage loans, Total amortized cost	1,262.9	1,054.3
Recorded investment 90 days or more past due and accruing	7.6	5.6
First liens \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	37.2	40.1
First liens \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	7.9	11.3
First liens \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	$ 10.6	$ 10.0

Recorded investment in financing receivables that are 90 days or more past due and still accruing.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

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Financing receivables that are current.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953401-111524

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Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(3))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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