Securities - Securities Available for Sale in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	$ 291,205	$ 323,469
Unrealized loss, Less than 12 months	(3,635)	(5,040)
Total Fair value	291,205	323,469
Total, unrealized loss	(3,635)	(5,040)
States of the U.S. and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	47,500	53,283
Unrealized loss, Less than 12 months	(1,410)	(1,661)
Total Fair value	47,500	53,283
Total, unrealized loss	(1,410)	(1,661)
U.S. Treasury and Government Sponsored Entities' Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	161,215	171,411
Unrealized loss, Less than 12 months	(1,436)	(2,221)
Total Fair value	161,215	171,411
Total, unrealized loss	(1,436)	(2,221)
Mortgage-Backed GSE Securities: Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	82,490	98,775
Unrealized loss, Less than 12 months	(789)	(1,158)
Total Fair value	82,490	98,775
Total, unrealized loss	$ (789)	$ (1,158)