Investment Securities - Gross Unrealized Losses on Debt Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$ 499,349	$ 289,271
Less than 12 months, Unrealized Loss	(11,801)	(2,211)
12 months or more, Fair Value	174,366	183,423
12 months or more, Unrealized Loss	(7,957)	(3,719)
Total Fair Value	673,715	472,694
Total Unrealized Loss	(19,758)	(5,930)
Less than 12 months, Fair Value, held to maturity	308,476	94,505
Less than 12 months, Unrealized Loss, held to maturity	(4,695)	(574)
12 months or more, Fair Value, held to maturity	92,388	98,004
12 months or more, Unrealized Loss, held to maturity	(3,273)	(1,352)
Total Fair Value, held to maturity	400,864	192,509
Total Unrealized Loss, held to maturity	(7,968)	(1,926)
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	434,302	284,367
Less than 12 months, Unrealized Loss	(10,345)	(2,176)
12 months or more, Fair Value	158,324	166,338
12 months or more, Unrealized Loss	(6,625)	(3,425)
Total Fair Value	592,626	450,705
Total Unrealized Loss	(16,970)	(5,601)
Less than 12 months, Fair Value, held to maturity	299,575	93,017
Less than 12 months, Unrealized Loss, held to maturity	(4,551)	(567)
12 months or more, Fair Value, held to maturity	89,809	95,367
12 months or more, Unrealized Loss, held to maturity	(3,186)	(1,322)
Total Fair Value, held to maturity	389,384	188,384
Total Unrealized Loss, held to maturity	(7,737)	(1,889)
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	65,047	4,904
Less than 12 months, Unrealized Loss	(1,456)	(35)
12 months or more, Fair Value	16,042	17,085
12 months or more, Unrealized Loss	(1,332)	(294)
Total Fair Value	81,089	21,989
Total Unrealized Loss	(2,788)	(329)
Less than 12 months, Fair Value, held to maturity	8,901	1,488
Less than 12 months, Unrealized Loss, held to maturity	(144)	(7)
12 months or more, Fair Value, held to maturity	2,579	2,637
12 months or more, Unrealized Loss, held to maturity	(87)	(30)
Total Fair Value, held to maturity	11,480	4,125
Total Unrealized Loss, held to maturity	$ (231)	$ (37)