Securities - Securities Available for Sale in Unrealized Loss Position (Detail) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	$ 8,272	$ 240,461
Unrealized loss, Less than 12 months	(17)	(2,496)
Fair value, 12 months or more		69,333
Unrealized loss, 12 months or more		(1,743)
Total Fair value	8,272	309,794
Total, unrealized loss	(17)	(4,239)
U.S. Treasury and Government Sponsored Entities' Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months		139,876
Unrealized loss, Less than 12 months		(1,654)
Fair value, 12 months or more		55,055
Unrealized loss, 12 months or more		(1,355)
Total Fair value		194,931
Total, unrealized loss		(3,009)
States of the U.S. and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	8,272
Unrealized loss, Less than 12 months	(17)
Total Fair value	8,272
Total, unrealized loss	$ (17)
Mortgage-Backed GSE Securities: Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months		100,585
Unrealized loss, Less than 12 months		(842)
Fair value, 12 months or more		14,278
Unrealized loss, 12 months or more		(388)
Total Fair value		114,863
Total, unrealized loss		$ (1,230)