Investment Securities - Schedule of Investment Securities' Gross Unrealized Losses and Fair Value by Investment Category (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	$ 521,105	$ 59,338
Available-for-sale securities, Less than 12 months, Unrealized Loss	6,269	236
Available-for-sale securities, 12 months or longer, Fair Value	34,795	271,416
Available-for-sale securities, 12 months or longer, Unrealized Loss	1,095	3,358
Available-for-sale securities, Total, Fair Value	555,900	330,754
Available-for-sale securities, Total, Unrealized Loss	7,364	3,594
Collateralized Mortgage Obligation (CMO) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	123,638	15,492
Available-for-sale securities, Less than 12 months, Unrealized Loss	1,244	108
Available-for-sale securities, 12 months or longer, Fair Value	28,247	61,630
Available-for-sale securities, 12 months or longer, Unrealized Loss	789	1,506
Available-for-sale securities, Total, Fair Value	151,885	77,122
Available-for-sale securities, Total, Unrealized Loss	2,033	1,614
Federal National Mortgage Association (FNMA) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	184,403
Available-for-sale securities, Less than 12 months, Unrealized Loss	2,472
Available-for-sale securities, 12 months or longer, Fair Value		103,207
Available-for-sale securities, 12 months or longer, Unrealized Loss		1,106
Available-for-sale securities, Total, Fair Value	184,403	103,207
Available-for-sale securities, Total, Unrealized Loss	2,472	1,106
Federal Home Loan Mortgage Corporation (FHLMC) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	63,566	23,901
Available-for-sale securities, Less than 12 months, Unrealized Loss	400	54
Available-for-sale securities, 12 months or longer, Fair Value	4,151	58,267
Available-for-sale securities, 12 months or longer, Unrealized Loss	221	478
Available-for-sale securities, Total, Fair Value	67,717	82,168
Available-for-sale securities, Total, Unrealized Loss	621	532
Government National Mortgage Association (GNMA) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	39,108
Available-for-sale securities, Less than 12 months, Unrealized Loss	93
Available-for-sale securities, 12 months or longer, Fair Value	2,397	48,312
Available-for-sale securities, 12 months or longer, Unrealized Loss	85	268
Available-for-sale securities, Total, Fair Value	41,505	48,312
Available-for-sale securities, Total, Unrealized Loss	178	268
U.S. Government and Government Sponsored Enterprises (GSE) [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	13,035	19,945
Available-for-sale securities, Less than 12 months, Unrealized Loss	15	74
Available-for-sale securities, Total, Fair Value	13,035	19,945
Available-for-sale securities, Total, Unrealized Loss	15	$ 74
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, Less than 12 months, Fair Value	97,355
Available-for-sale securities, Less than 12 months, Unrealized Loss	2,045
Available-for-sale securities, Total, Fair Value	97,355
Available-for-sale securities, Total, Unrealized Loss	$ 2,045