Securities - Additional Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Proceeds from sales of securities available for sale	$ 598,740,000	$ 705,238,000
Gross realized gains from securities	5,500,000	8,400,000
Gross realized losses from securities		48,000
Sales of securities held to maturity	0	0
Unrealized gains on trading securities	93,000	$ 156,000
Fair value of other marketable securities	8,000,000		$ 7,200,000
Fair value of other non-marketable securities	1,900,000		2,000,000
U.S. Government and Other Public Deposit [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Pledged securities for deposits	5,500,000,000		5,900,000,000
Federal Reserve Discount Window [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Pledged securities for deposits	$ 1,500,000,000		$ 1,600,000,000