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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	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	$ 15.0	$ (11.5)	$ (2.8)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	(5.2)	(8.2)	0.7
Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(0.9)	12.4
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.4	0.1
Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	15.9	(23.9)	(2.8)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	(5.6)	(8.3)	0.7
Commission Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	10.4	(24.0)	(3.3)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	(8.7)	(9.1)	0.7
Commission Revenue [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	10.4	(24.0)	(3.3)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	(8.7)	(9.1)	0.7
Compensation Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	3.2	0.1	0.3
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	1.8	0.5
Compensation Expense [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	3.2	0.1	0.3
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	1.8	0.5
Operating Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	2.3		0.2
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	1.3	0.3
Operating Expense [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	2.3		$ 0.2
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	1.3	0.3
Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(0.9)	12.4
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.4	0.1
Interest Expense [Member] | Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(0.9)	12.4
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	$ 0.4	$ 0.1

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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
-Subparagraph (b)
-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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