Investments - Non-Accrual Mortgage Loans (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	$ 16.1	$ 18.7
Past due	59.1	39.6
Current	12,675.8	12,351.4
Mortgage loans, Total amortized cost	12,734.9	12,391.0	$ 12,126.6
Recorded investment 90 days or more past due and accruing	7.2	3.4
30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	33.6	22.5
60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	12.0	6.5
90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	13.5	10.6
Commercial mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	11,556.0	11,265.3	10,991.7
Brick and mortar
Mortgage loan non-accrual and aging disclosures
Current	11,241.7	10,921.0
Mortgage loans, Total amortized cost	11,241.7	10,921.0
Credit tenant loans
Mortgage loan non-accrual and aging disclosures
Current	314.3	344.3
Mortgage loans, Total amortized cost	314.3	344.3
Residential mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	1,178.9	1,125.7	$ 1,134.9
Home equity
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	9.9	10.8
Past due	3.4	3.6
Current	191.2	215.2
Mortgage loans, Total amortized cost	194.6	218.8
Home equity \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	1.7	2.0
Home equity \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	0.7	1.0
Home equity \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	1.0	0.6
First liens
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	6.2	7.9
Past due	55.7	36.0
Current	928.6	870.9
Mortgage loans, Total amortized cost	984.3	906.9
Recorded investment 90 days or more past due and accruing	7.2	3.4
First liens \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	31.9	20.5
First liens \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	11.3	5.5
First liens \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	$ 12.5	$ 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=49124121&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=49124121&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))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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