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Derivatives and Hedging Activity - Summary of Amounts of Derivative Gains (Losses) Recognized In Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	$ 8.5	$ 6.2
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	1.8	(2.4)
Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	4.2	0.8
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.2
Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	4.3	5.4
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	1.6	(2.4)
Commission Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	4.7	2.4
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.7	(2.8)
Commission Revenue [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	4.7	2.4
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.7	(2.8)
Compensation Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(0.2)	1.8
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.5	0.2
Compensation Expense [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(0.2)	1.8
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.5	0.2
Operating Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(0.2)	1.2
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.4	0.2
Operating Expense [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(0.2)	1.2
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.4	0.2
Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	4.2	0.8
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.2
Interest Expense [Member] | Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	4.2	$ 0.8
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	$ 0.2

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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
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-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959

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- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

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