Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 845,873	$ 850,699
Gross Unrealized Gains	4,499	1,170
Gross Unrealized Losses	(8,666)	(34,754)
Fair Value	841,706	817,115
Collateralized Mortgage Obligation ("CMO") [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	163,781	103,064
Gross Unrealized Gains	482	28
Gross Unrealized Losses	(2,185)	(5,535)
Fair Value	162,078	97,557
Federal National Mortgage Association ("FNMA") [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	321,960	382,909
Gross Unrealized Gains	735	20
Gross Unrealized Losses	(3,371)	(15,801)
Fair Value	319,324	367,128
Federal Home Loan Mortgage Corporation ("FHLMC") [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	127,454	129,460
Gross Unrealized Gains	244	29
Gross Unrealized Losses	(1,406)	(4,994)
Fair Value	126,292	124,495
Government National Mortgage Association ("GNMA") [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	84,436	97,830
Gross Unrealized Gains	499	743
Gross Unrealized Losses	(525)	(2,981)
Fair Value	84,410	95,592
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	111,256	105,354
Gross Unrealized Gains	2,484	257
Gross Unrealized Losses	(1,159)	(5,426)
Fair Value	112,581	100,185
U.S. Government and Government Sponsored Enterprises ("GSE") [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,986	32,082
Gross Unrealized Gains	55	93
Gross Unrealized Losses	(20)	(17)
Fair Value	$ 37,021	$ 32,158