Investments - Non-Accrual Mortgage Loans (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	$ 74.8	$ 14.4
Past due	72.8	65.8
Current	13,991.3	13,209.3
Mortgage loans, Total amortized cost	14,064.1	13,275.1	$ 13,134.5
Recorded investment 90 days or more past due and accruing	10.2	5.6
30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	52.5	42.0
60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	6.4	12.4
90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	13.9	11.4
Commercial mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	12,773.5	12,055.2	11,919.6
Brick and mortar
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	64.1
Current	12,535.0	11,795.2
Mortgage loans, Total amortized cost	12,535.0	11,795.2
Credit tenant loans
Mortgage loan non-accrual and aging disclosures
Current	238.5	260.0
Mortgage loans, Total amortized cost	238.5	260.0
Residential mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	1,290.6	1,219.9	$ 1,214.9
Home equity
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	6.9	8.8
Past due	2.7	4.4
Current	123.0	161.2
Mortgage loans, Total amortized cost	125.7	165.6
Home equity \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	1.3	1.9
Home equity \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	0.6	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	3.8	5.6
Past due	70.1	61.4
Current	1,094.8	992.9
Mortgage loans, Total amortized cost	1,164.9	1,054.3
Recorded investment 90 days or more past due and accruing	10.2	5.6
First liens \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	51.2	40.1
First liens \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	5.8	11.3
First liens \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	$ 13.1	$ 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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+ 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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+ 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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