Investment Securities - Amortized Cost and Estimated Fair Values of Investments in Debt Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 754,870	$ 731,292
Gross Unrealized Gains	783	2,583
Gross Unrealized Losses	(19,758)	(5,930)
Estimated Fair Value	735,895	727,945
Amortized Cost, Held to Maturity	496,035	514,844
Gross Unrealized Gains, Held to Maturity	572	5,247
Gross Unrealized Losses, Held to Maturity	(7,968)	(1,926)
Estimated Fair Value, Held to Maturity	488,639	518,165
Obligations of U.S. Government Corporations and Agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	633,310	609,695
Gross Unrealized Gains	317	695
Gross Unrealized Losses	(16,970)	(5,601)
Estimated Fair Value	616,657	604,789
Amortized Cost, Held to Maturity	481,457	500,271
Gross Unrealized Gains, Held to Maturity	533	5,101
Gross Unrealized Losses, Held to Maturity	(7,737)	(1,889)
Estimated Fair Value, Held to Maturity	474,253	503,483
Obligations of States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	121,560	121,597
Gross Unrealized Gains	466	1,888
Gross Unrealized Losses	(2,788)	(329)
Estimated Fair Value	119,238	123,156
Amortized Cost, Held to Maturity	14,578	14,573
Gross Unrealized Gains, Held to Maturity	39	146
Gross Unrealized Losses, Held to Maturity	(231)	(37)
Estimated Fair Value, Held to Maturity	$ 14,386	$ 14,682