Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2016 USD ($) Investment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Investment Security Losses, net	$ (3,200,000)	$ (300,000)	$ (4,300,000)
Note: Changes in Other-Than-Temporary-Impairment (OTTI) Losses	3,700,000	0	4,900,000
Other Than Temporary Impairment Losses Recognized In Earnings	3,700,000	0	4,200,000
Gross proceeds from sale of securities	828,900,000	262,100,000	851,800,000
Gross realized securities gains	700,000	200,000	2,800,000
Gross realized securities losses	$ 200,000	500,000	$ 2,900,000
Number of securities in an unrealized loss position | Investment	1,154
Total Fair Value	$ 22,040,200,000	23,548,600,000
Total Unrealized Losses	176,000,000	$ 140,500,000
Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	70,600,000
Other | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	35,900,000
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total Unrealized Losses	$ 25,200,000
Percent of corporate debt portfolio	34.00%