Securities - Summary of Amortized Cost and Estimated Fair Values of Securities Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 509,631	$ 536,409
Gross Unrealized Gains	6,817	9,916
Gross Unrealized Losses	(807)	(199)
Estimated Fair Value, Total	515,641	546,126
U.S. treasuries [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,946
Gross Unrealized Gains
Gross Unrealized Losses	(85)
Estimated Fair Value, Total	29,861
U.S. Federal agencies [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	386,391	452,554
Gross Unrealized Gains	1,399	3,654
Gross Unrealized Losses	(505)	(164)
Estimated Fair Value, Total	387,285	456,044
Mortgage backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	32,057	19,100
Gross Unrealized Gains	625	825
Gross Unrealized Losses	(4)	(1)
Estimated Fair Value, Total	32,678	19,924
States and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,116	53,912
Gross Unrealized Gains	1,290	2,875
Gross Unrealized Losses	(72)	(20)
Estimated Fair Value, Total	50,334	56,767
Other securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	12,121	10,843
Gross Unrealized Gains	3,503	2,562
Gross Unrealized Losses	(141)	(14)
Estimated Fair Value, Total	$ 15,483	$ 13,391