Securities - Narrative (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Investment	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Trading Securities and Other Trading Assets [Line Items]
Transferred from available for sale to held to maturity	$ 1,400,000,000
Investment security losses, net	(1,600,000)	$ (3,200,000)	$ (300,000)
Changes in other-than-temporary-impairment (OTTI) losses	200,000	3,700,000	0
Gross proceeds from sale of securities	2,200,000,000	828,900,000	262,100,000
Gross realized securities gains	200,000	700,000	200,000
Gross realized securities losses	$ 1,600,000	200,000	$ 500,000
Number of securities in an unrealized loss position | Investment	1,285
Total fair value	$ 27,562,000,000	22,040,200,000
Total unrealized losses	249,400,000	$ 176,000,000
Government Sponsored Agency
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	108,600,000
US Government Securities
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	32,100,000
Other | Community Reinvestment Act CRA
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	45,200,000
Corporate Debt
Schedule of Trading Securities and Other Trading Assets [Line Items]
Total unrealized losses	$ 23,300,000
Percent of corporate debt portfolio	36.00%