Derivative Financial Instruments (Fair and Notional Values of Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Notional Amount	$ 14,681.1	$ 13,128.8
Asset Fair Value	71.5	111.6
Liability Fair Value	(80.0)	(68.8)
Total Return Swap (TRS)
Derivatives, Fair Value [Line Items]
Notional Amount	174.3	587.5
Qualifying Hedges
Derivatives, Fair Value [Line Items]
Notional Amount	976.6	817.9
Asset Fair Value	1.1	16.9
Liability Fair Value	(7.2)	0.0
Qualifying Hedges \| Foreign currency forward contracts — net investment hedges \| Foreign currency forward contracts
Derivatives, Fair Value [Line Items]
Notional Amount	976.6	817.9
Asset Fair Value	1.1	16.9
Liability Fair Value	(7.2)	0.0
Non-Qualifying Hedges
Derivatives, Fair Value [Line Items]
Notional Amount	13,704.5	12,310.9
Asset Fair Value	70.4	94.7
Liability Fair Value	(72.8)	(68.8)
Non-Qualifying Hedges \| Interest rate swaps
Derivatives, Fair Value [Line Items]
Notional Amount	6,623.3	5,309.2
Asset Fair Value	62.8	63.0
Liability Fair Value	(32.9)	(50.1)
Non-Qualifying Hedges \| Options \| Written options
Derivatives, Fair Value [Line Items]
Notional Amount	2,736.3	2,626.5
Asset Fair Value	0.0	0.1
Liability Fair Value	(0.6)	(1.0)
Non-Qualifying Hedges \| Options \| Purchased options
Derivatives, Fair Value [Line Items]
Notional Amount	2,545.1	2,129.6
Asset Fair Value	0.6	1.0
Liability Fair Value	0.0	(0.1)
Non-Qualifying Hedges \| Foreign currency forward contracts
Derivatives, Fair Value [Line Items]
Notional Amount	1,337.0	1,329.8
Asset Fair Value	6.9	30.2
Liability Fair Value	(25.5)	(6.0)
Non-Qualifying Hedges \| Total Return Swap (TRS)
Derivatives, Fair Value [Line Items]
Notional Amount	174.3	587.5
Asset Fair Value	0.0	0.0
Liability Fair Value	(13.7)	(11.3)
Non-Qualifying Hedges \| Equity Warrants
Derivatives, Fair Value [Line Items]
Notional Amount	1.0	1.0
Asset Fair Value	0.0	0.2
Liability Fair Value	0.0	0.0
Non-Qualifying Hedges \| Interest Rate Lock Commitments
Derivatives, Fair Value [Line Items]
Notional Amount	12.5	20.7
Asset Fair Value	0.1	0.1
Liability Fair Value	0.0	(0.1)
Non-Qualifying Hedges \| Forward Sale Commitments on Agency MBS
Derivatives, Fair Value [Line Items]
Notional Amount	7.5	39.0
Asset Fair Value	0.0	0.1
Liability Fair Value	0.0	0.0
Non-Qualifying Hedges \| Credit derivatives
Derivatives, Fair Value [Line Items]
Notional Amount	267.5	267.6
Asset Fair Value	0.0	0.0
Liability Fair Value	$ (0.1)	$ (0.2)

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
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-URI http://asc.fasb.org/extlink&oid=82924980&loc=SL51823488-111719

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4D
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=SL51823488-111719

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313

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