Securities Available-for-Sale - Amortized Cost and Fair Value of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 658,006	$ 650,782
Fair Value	677,221	660,963
Obligations of the U.S. Treasury and U.S. government corporations and agencies, and obligations of states and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less, Amortized Cost	46,963
Due after one year through five years, Amortized Cost	220,193
Due after five years through ten years, Amortized Cost	66,133
Due after ten years, Amortized Cost	75,333
Available-for-sale Securities, Debt Maturities, Amortized Cost	408,622
Due in one year or less, Fair Value	47,106
Due after one year through five years, Fair Value	225,234
Due after five years through ten years, Fair Value	68,646
Due after ten years, Fair Value	80,003
Fair Value, Debt securities	420,989
Collateralized Mortgage Obligations of U.S. Government Corporations and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	134,502	128,458
Fair Value	137,330	128,835
Residential Mortgage-Backed Securities of U.S. Government Corporations and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,858	39,185
Fair Value	39,285	40,125
Commercial Mortgage-Backed Securities of U.S. Government Corporations and Agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	69,445	69,697
Fair Value	70,664	69,204
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	650,427	643,203
Fair Value	668,268	651,294
Marketable Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,579	7,579
Fair Value	$ 8,953	$ 9,669