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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	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	$ (0.2)	$ 6.4
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	3.1	(4.1)
Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	5.9	(3.0)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.5
Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(6.1)	9.4
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	2.6	(4.1)
Commission Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(2.5)	6.0
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	1.1	(5.3)
Commission Revenue [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(2.5)	6.0
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	1.1	(5.3)
Compensation Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(2.1)	2.0
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.9	0.7
Compensation Expense [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(2.1)	2.0
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.9	0.7
Operating Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(1.5)	1.4
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.6	0.5
Operating Expense [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	(1.5)	1.4
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.6	0.5
Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	5.9	(3.0)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	0.5
Interest Expense [Member] | Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative gains (losses) recognized in accumulated other comprehensive loss	5.9	$ (3.0)
Derivative gains (losses) reclassified from accumulated other comprehensive loss into income (effective portion)	$ 0.5

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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.

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Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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