Other Operating Expense - (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components Of Other Operating Expense [Line Items]
Business Promotion	$ 95.4	$ 83.6	$ 85.1
FDIC Insurance Premiums	34.7	31.7	25.2
Staff Related	42.8	43.0	40.5
Other Intangibles Amortization	11.4	8.8	10.9
Other Expenses	147.3	197.3	166.3
Total Other Operating Expense	$ 331.6	364.4	328.0
Total NAV funds			$ (45.8)
Net Asset Value (in dollars per share)			$ 1.00
Agreement To Settle Securities
Components Of Other Operating Expense [Line Items]
Litigation expense		50.0
Contractual Modifications
Components Of Other Operating Expense [Line Items]
Litigation expense		$ 18.6