Investments - Non-Accrual Mortgage Loans (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	$ 17.5	$ 18.7
Past due	47.3	39.6
Current	12,380.7	12,351.4
Mortgage loans, Total amortized cost	12,428.0	12,391.0	$ 11,956.4
Recorded investment 90 days or more past due and accruing	6.2	3.4
30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	23.2	22.5
60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	10.8	6.5
90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	13.3	10.6
Commercial mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	11,267.7	11,265.3	10,820.8
Brick and mortar
Mortgage loan non-accrual and aging disclosures
Current	10,934.1	10,921.0
Mortgage loans, Total amortized cost	10,934.1	10,921.0
Credit tenant loans
Mortgage loan non-accrual and aging disclosures
Current	333.6	344.3
Mortgage loans, Total amortized cost	333.6	344.3
Residential mortgage loans
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Total amortized cost	1,160.3	1,125.7	$ 1,135.6
Home equity
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	10.2	10.8
Past due	3.2	3.6
Current	204.1	215.2
Mortgage loans, Total amortized cost	207.3	218.8
Home equity \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	1.8	2.0
Home equity \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	0.5	1.0
Home equity \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	0.9	0.6
First liens
Mortgage loan non-accrual and aging disclosures
Mortgage loans, Non-accrual status	7.3	7.9
Past due	44.1	36.0
Current	908.9	870.9
Mortgage loans, Total amortized cost	953.0	906.9
Recorded investment 90 days or more past due and accruing	6.2	3.4
First liens \| 30 to 59 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	21.4	20.5
First liens \| 60 to 89 Days Past Due
Mortgage loan non-accrual and aging disclosures
Past due	10.3	5.5
First liens \| 90 Days or More Past Due
Mortgage loan non-accrual and aging disclosures
Past due	$ 12.4	$ 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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