Securities - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2015 USD ($) Investment	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Losses, net	$ (0.3)	$ (4.3)	$ (1.5)
Net impairment losses recognized in earnings	0.0	4.2	0.0
Gross proceeds from sale of securities	262.1	851.8	506.3
Gross realized securities gains	0.2	2.8	0.8
Gross realized securities losses	$ 0.5	2.9	$ 2.3
Number of securities in an unrealized loss position | Investment	897
Total Fair Value	$ 23,548.6	10,410.8
Total Unrealized Losses	140.5	$ 87.6
U.S. Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	46.5
Others | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	36.9
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	$ 33.1
Percent of corporate debt portfolio	35.00%