Amortized Cost and Estimated Fair Value of Available for Sale Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 320,030	$ 311,747
Gross Unrealized Gains	2,787	5,081
Gross Unrealized Losses	(2,900)	(214)
Estimated Fair Value	319,917	316,614
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	102
Gross Unrealized Losses	(1)
Estimated Fair Value	101
Obligations of U.S. government and agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	92,128	73,183
Gross Unrealized Gains	247	796
Gross Unrealized Losses	(1,431)	(107)
Estimated Fair Value	90,944	73,872
Obligations of state and political subdivisions
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	40,273	30,244
Gross Unrealized Gains	115	199
Gross Unrealized Losses	(323)	(59)
Estimated Fair Value	40,065	30,384
Mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	123,161	128,537
Gross Unrealized Gains	1,909	3,302
Gross Unrealized Losses	(787)	(13)
Estimated Fair Value	124,283	131,826
Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	47,412	62,116
Gross Unrealized Gains	268	622
Gross Unrealized Losses	(357)	(35)
Estimated Fair Value	47,323	62,703
Other Investees
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	16,954	17,667
Gross Unrealized Gains	248	162
Gross Unrealized Losses	(1)
Estimated Fair Value	$ 17,201	$ 17,829