Derivative Financial Instruments (Notional and Fair Value of Designated Risk Management Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Fair value asset	$ 2,576.5	$ 3,047.6
Fair value liability	2,327.6	2,767.6
Asset And Liability Management
Derivative [Line Items]
Notional Value	10,431.5	9,942.9
Fair value asset	79.7	494.8
Fair value liability	180.2	42.2
Asset And Liability Management \| Fair Value Hedges \| Senior Notes and Long-Term Subordinated Debt \| Interest Rate Swap Contracts
Derivative [Line Items]
Notional Value	1,250.0	1,250.0
Fair value asset	18.0	71.8
Fair value liability	4.1	3.3
Asset And Liability Management \| Fair Value Hedges \| Available for Sale Investment Securities \| Interest Rate Swap Contracts
Derivative [Line Items]
Notional Value	3,430.1	3,873.4
Fair value asset	17.4	88.3
Fair value liability	11.0	16.8
Asset And Liability Management \| Cash Flow Hedges \| Forecasted Foreign Currency Denominated Transactions \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	431.2	329.3
Fair value asset	11.7	8.5
Fair value liability	8.5	7.8
Asset And Liability Management \| Cash Flow Hedges \| Foreign Currency Denominated Investment Securities \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	1,439.8	1,431.6
Fair value asset	27.4	151.5
Fair value liability	0.0	0.8
Asset And Liability Management \| Cash Flow Hedges \| Available for Sale Investment Securities \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	1,025.0	975.0
Fair value asset	0.2	0.1
Fair value liability	1.7	2.7
Asset And Liability Management \| Net Investment Hedges \| Net Investments in Non-U.S. Affiliates \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	2,855.4	2,083.6
Fair value asset	5.0	174.6
Fair value liability	$ 154.9	$ 10.8

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
-Paragraph 4D
-Subparagraph (c)(2)
-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
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-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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