Contractual Maturity Information for Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Due in 1 year or less, Amortized Cost	$ 6,919	$ 2,275
Due after 1 year through 5 years, Amortized Cost	106,654	100,648
Due after 5 years through 10 years, Amortized Cost	349,733	372,234
Due after 10 years, Amortized Cost	741,276	785,857
Total state and political subdivisions, Amortized Cost	1,204,582	1,261,014
Due in 1 year or less, Gross Unrealized Gains		3
Due after 1 year through 5 years, Gross Unrealized Gains	226	3,111
Due after 5 years through 10 years, Gross Unrealized Gains	3,376	5,006
Due after 10 years, Gross Unrealized Gains	5,280	6,952
Total state and political subdivisions, Gross Unrealized Gains	8,882	15,072
Due in 1 year or less, Gross Unrealized Losses	(52)	(24)
Due after 1 year through 5 years, Gross Unrealized Losses	(9,188)	(2,834)
Due after 5 years through 10 years, Gross Unrealized Losses	(20,134)	(14,117)
Due after 10 years, Gross Unrealized Losses	(80,153)	(51,664)
Total state and political subdivisions, Gross Unrealized Losses	(109,527)	(68,639)
Due in 1 year or less, Fair Value	6,867	2,254
Due after 1 year through 5 years, Fair Value	97,692	100,925
Due after 5 years through 10 years, Fair Value	332,975	363,123
Due after 10 years, Fair Value	666,403	741,145
Total state and political subdivisions, Fair Value	$ 1,103,937	$ 1,207,447

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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