Contractual Maturity Information for Securities Held to Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Investments Debt And Equity Securities [Abstract]
Due in 1 year or less, Amortized Cost	$ 7,434	$ 2,275
Due after 1 year through 5 years, Amortized Cost	101,824	100,648
Due after 5 years through 10 years, Amortized Cost	372,672	372,234
Due after 10 years, Amortized Cost	717,184	785,857
Total state and political subdivisions, Amortized Cost	1,199,114	1,261,014
Due in 1 year or less, Gross Unrealized Gains		3
Due after 1 year through 5 years, Gross Unrealized Gains	237	3,111
Due after 5 years through 10 years, Gross Unrealized Gains	4,639	5,006
Due after 10 years, Gross Unrealized Gains	5,200	6,952
Total state and political subdivisions, Gross Unrealized Gains	10,076	15,072
Due in 1 year or less, Gross Unrealized Losses	(1,027)	(24)
Due after 1 year through 5 years, Gross Unrealized Losses	(9,363)	(2,834)
Due after 5 years through 10 years, Gross Unrealized Losses	(23,328)	(14,117)
Due after 10 years, Gross Unrealized Losses	(83,832)	(51,664)
Total state and political subdivisions, Gross Unrealized Losses	(117,550)	(68,639)
Due in 1 year or less, Fair Value	6,407	2,254
Due after 1 year through 5 years, Fair Value	92,698	100,925
Due after 5 years through 10 years, Fair Value	353,983	363,123
Due after 10 years, Fair Value	638,552	741,145
Total state and political subdivisions, Fair Value	$ 1,091,640	$ 1,207,447

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