Securities Available-for-Sale - Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 649,198	$ 650,782
Fair Value	671,128	660,963
Obligations of the U.S. Treasury, U.S. government corporations and agencies, and obligations of states and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	52,302
Due after one year through five years, Amortized Cost	216,729
Due after five years through ten years, Amortized Cost	55,893
Due after ten years, Amortized Cost	60,508
Available-for-sale Securities, Debt Maturities, Amortized Cost	385,432
Due in one year or less, Fair Value	52,439
Due after one year through five years, Fair Value	222,114
Due after five years through ten years, Fair Value	58,699
Due after ten years, Fair Value	65,264
Fair Value, Debt securities	398,516
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	134,675	128,458
Fair Value	137,460	128,835
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	34,685	39,185
Fair Value	36,285	40,125
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	86,827	69,697
Fair Value	88,499	69,204
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	641,619	643,203
Fair Value	660,760	651,294
Marketable Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Fair Value	$ 10,368	$ 9,669