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, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	[1]	$ (37.1)	$ (15.7)
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income Loss into Earnings		(5.2)	5.7
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		1.3
Commission revenue [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	[1]	9.9	(6.4)
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income Loss into Earnings		(1.6)	2.3
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		(0.8)
Compensation expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income Loss into Earnings		(1.4)	1.3
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		1.2
Operating expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income Loss into Earnings		(1.0)	1.0
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		0.9
Interest expense [Member] | Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	[1]	(47.0)	(9.3)
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Income into Income Loss into Earnings		$ (1.2)	$ 1.1

[1] During 2019, the amount excluded from the assessment of hedge effectiveness for our foreign exchange contracts recognized in accumulated other comprehensive loss were a loss of $0.2 million.