Goodwill and Other Intangibles - Goodwill (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Roll Forward]
Beginning Balance	$ 519.4	$ 526.4
Goodwill Acquired	78.3	11.8
Measurement Period Adjustments	(1.3)
Foreign Exchange Rates	9.2	(18.8)
Ending Balance	605.6	519.4
CORPORATE & INSTITUTIONAL SERVICES
Goodwill [Roll Forward]
Beginning Balance	448.4	455.1
Goodwill Acquired	78.3	11.8
Measurement Period Adjustments	(1.3)
Foreign Exchange Rates	9.1	(18.5)
Ending Balance	534.5	448.4
WEALTH MANAGEMENT
Goodwill [Roll Forward]
Beginning Balance	71.0	71.3
Goodwill Acquired	0.0	0.0
Measurement Period Adjustments	0.0
Foreign Exchange Rates	0.1	(0.3)
Ending Balance	$ 71.1	$ 71.0