Investments - Non-Accrual Mortgage Loans (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	$ 12.8	$ 14.4
Past due	64.5	65.8
Current	13,368.8	13,209.3
Mortgage loans, Total amortized cost	13,433.3	13,275.1	$ 12,428.0
Recorded investment 90 days or more past due and accruing	6.5	5.6
30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	41.1	42.0
60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	12.0	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	12,165.7	12,055.2	11,267.7
Brick and mortar
Mortgage loan non-accrual and aging disclosures
Current	11,912.8	11,795.2
Mortgage loans, Total amortized cost	11,912.8	11,795.2
Credit tenant loans
Mortgage loan non-accrual and aging disclosures
Current	252.9	260.0
Mortgage loans, Total amortized cost	252.9	260.0
Residential mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	1,267.6	1,219.9	$ 1,160.3
Home equity
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	8.7	8.8
Past due	4.2	4.4
Current	148.1	161.2
Mortgage loans, Total amortized cost	152.3	165.6
Home equity \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	1.8	1.9
Home equity \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	0.9	1.1
Home equity \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	1.5	1.4
First liens
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	4.1	5.6
Past due	60.3	61.4
Current	1,055.0	992.9
Mortgage loans, Total amortized cost	1,115.3	1,054.3
Recorded investment 90 days or more past due and accruing	6.5	5.6
First liens \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	39.3	40.1
First liens \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	11.1	11.3
First liens \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	$ 9.9	$ 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
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: 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=66008989&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=66008989&loc=SL6953401-111524

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Recorded Investment
-URI http://asc.fasb.org/extlink&oid=65880944

Reference 2: 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=66008989&loc=SL6953401-111524

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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))
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