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, 2020	Dec. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	[1]	$ (97.1)	$ (37.1)
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Loss into Earnings		(6.3)	(5.2)
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		2.3	1.3
Interest expense [Member] | Interest Rate Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	[1]	(92.5)	(47.0)
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Loss into Earnings		(1.1)	(1.2)
Commission revenue [Member] | Foreign Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Recognized in Accumulated Other Comprehensive Loss	[1]	(4.6)	9.9
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Loss into Earnings		(2.4)	(1.6)
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		(0.4)	(0.8)
Compensation expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Loss into Earnings		(1.6)	(1.4)
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		1.6	1.2
Operating expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from Accumulated Other Comprehensive Loss into Earnings		(1.2)	(1.0)
Amount of Gain (Loss) Recognized in Earnings Related to Amount Excluded from Effectiveness Testing		$ 1.1	$ 0.9

[1] During 2020, the amount excluded from the assessment of hedge effectiveness for our foreign exchange contracts recognized in accumulated other comprehensive loss was a loss of $1.3 million.