Derivative Financial Instruments - Notional and Fair Values of Designated Risk Management Derivative Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Fair Value Asset	$ 2,203.3	$ 3,047.6
Fair Value Liability	2,003.1	2,767.6
Asset And Liability Management
Derivative [Line Items]
Notional Value	11,698.4	9,942.9
Fair Value Asset	60.9	494.8
Fair Value Liability	212.0	42.2
Asset And Liability Management \| Fair Value Hedges \| Senior Notes and Long- Term Subordinated Debt \| Interest Rate Swaps
Derivative [Line Items]
Notional Value	1,050.0	1,250.0
Fair Value Asset	16.0	71.8
Fair Value Liability	3.7	3.3
Asset And Liability Management \| Fair Value Hedges \| Securities Available for Sale \| Interest Rate Swaps
Derivative [Line Items]
Notional Value	3,423.1	3,873.4
Fair Value Asset	15.7	88.3
Fair Value Liability	14.5	16.8
Asset And Liability Management \| Cash Flow Hedges \| Forecasted Foreign Currency Denominated Transactions \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	436.2	329.3
Fair Value Asset	13.5	8.5
Fair Value Liability	6.4	7.8
Asset And Liability Management \| Cash Flow Hedges \| Foreign Currency Denominated Investment Securities \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	2,852.8	1,431.6
Fair Value Asset	14.9	151.5
Fair Value Liability	6.6	0.8
Asset And Liability Management \| Cash Flow Hedges \| Securities Available for Sale \| Interest Rate Option Contracts
Derivative [Line Items]
Notional Value	925.0	975.0
Fair Value Asset	0.2	0.1
Fair Value Liability	1.2	2.7
Asset And Liability Management \| Net Investment Hedges \| Net Investments in Non-U.S. Affiliates \| Foreign Exchange Contracts
Derivative [Line Items]
Notional Value	3,011.3	2,083.6
Fair Value Asset	0.6	174.6
Fair Value Liability	$ 179.6	$ 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)
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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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-Name Accounting Standards Codification
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