Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Available-for-Sale Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 878,664	$ 850,699
Gross Unrealized Gains	2,058	1,170
Gross Unrealized Losses	(21,471)	(34,754)
Fair Value	859,251	817,115
Collateralized Mortgage Obligation ("CMO") [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	114,037	103,064
Gross Unrealized Gains	30	28
Gross Unrealized Losses	(3,984)	(5,535)
Fair Value	110,083	97,557
Federal National Mortgage Association ("FNMA") [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	380,387	382,909
Gross Unrealized Gains	87	20
Gross Unrealized Losses	(10,004)	(15,801)
Fair Value	370,470	367,128
Federal Home Loan Mortgage Corporation ("FHLMC") [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	136,274	129,460
Gross Unrealized Gains	26	29
Gross Unrealized Losses	(3,401)	(4,994)
Fair Value	132,899	124,495
Government National Mortgage Association ("GNMA") [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	104,361	97,830
Gross Unrealized Gains	367	743
Gross Unrealized Losses	(1,587)	(2,981)
Fair Value	103,141	95,592
State and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	111,583	105,354
Gross Unrealized Gains	1,475	257
Gross Unrealized Losses	(2,483)	(5,426)
Fair Value	110,575	100,185
U.S. Government and government sponsored enterprises ("GSE") [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,022	32,082
Gross Unrealized Gains	73	93
Gross Unrealized Losses	(12)	(17)
Fair Value	$ 32,083	$ 32,158