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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, 2019	Mar. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Earnings	[1]	$ (17.2)	$ 8.5
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Earnings into Earnings		(0.7)	1.8
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		0.4
Commission revenue [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Earnings	[1]	3.7	4.3
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Earnings into Earnings		(0.1)	0.7
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		(0.2)
Compensation expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Earnings into Earnings		(0.2)	0.5
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		0.4
Operating expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Earnings into Earnings		(0.1)	0.4
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		0.2
Interest expense [Member] | Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Earnings	[1]	(20.9)	4.2
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Earnings into Earnings		(0.3)	$ 0.2
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		$ 0.0
[1] For the three-month period ended March 31, 2019, the amount excluded from the assessment of hedge effectiveness for our foreign exchange contracts recognized in Other comprehensive earnings was a loss of $0.1 million.

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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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