Derivative Financial Instruments (Notional and Fair Value of Designated Risk Management Derivative Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Fair value asset	$ 1,685.0	$ 2,203.3
Fair value liability	1,652.2	2,003.1
Asset And Liability Management
Derivative [Line Items]
Notional Value	11,197.3	11,698.4
Fair value asset	79.6	60.9
Fair value liability	76.8	212.0
Asset And Liability Management \| Fair Value Hedges \| Senior Notes and Long-Term Subordinated Debt \| Interest Rate Swap Contracts
Derivative [Line Items]
Notional Value	1,050.0	1,050.0
Fair value asset	9.4	16.0
Fair value liability	4.2	3.7
Asset And Liability Management \| Fair Value Hedges \| Available for Sale Debt Securities \| Interest Rate Swap Contracts
Derivative [Line Items]
Notional Value	3,307.4	3,423.1
Fair value asset	16.9	15.7
Fair value liability	11.5	14.5
Asset And Liability Management \| Cash Flow Hedges \| Forecasted Foreign Currency Denominated Transactions \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	465.3	436.2
Fair value asset	12.5	13.5
Fair value liability	7.9	6.4
Asset And Liability Management \| Cash Flow Hedges \| Foreign Currency Denominated Debt Securities \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	2,785.8	2,852.8
Fair value asset	6.8	14.9
Fair value liability	48.3	6.6
Asset And Liability Management \| Cash Flow Hedges \| Available for Sale Debt Securities \| Interest Rate Contracts
Derivative [Line Items]
Notional Value	925.0	925.0
Fair value asset	0.2	0.2
Fair value liability	2.3	1.2
Asset And Liability Management \| Net Investment Hedges \| Net Investments in Non-U.S. Affiliates \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	2,663.8	3,011.3
Fair value asset	33.8	0.6
Fair value liability	$ 2.6	$ 179.6

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
-SubTopic 20
-Section 55
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313

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