Securities - Securities Available for Sale in Unrealized Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	$ 72,398	$ 4,625
Unrealized loss, Less than 12 months	(196)	(40)
Fair value, 12 months or more	164,658	407,929
Unrealized loss, 12 months or more	(2,176)	(7,114)
Total Fair value	237,056	412,554
Total, unrealized loss	(2,372)	(7,154)
U.S. Treasury and Government Sponsored Entities' Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	41,547
Unrealized loss, Less than 12 months	(104)
Fair value, 12 months or more	55,810	214,495
Unrealized loss, 12 months or more	(762)	(4,452)
Total Fair value	97,357	214,495
Total, unrealized loss	(866)	(4,452)
Mortgage-Backed GSE Securities: Residential [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, Less than 12 months	30,851	4,625
Unrealized loss, Less than 12 months	(92)	(40)
Fair value, 12 months or more	108,848	193,434
Unrealized loss, 12 months or more	(1,414)	(2,662)
Total Fair value	139,699	198,059
Total, unrealized loss	$ (1,506)	$ (2,702)