Amortized Cost and Estimated Fair Values of Investments Held-To-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 215,814	$ 178,465
Gross Unrealized Gains	6,362	5,703
Gross Unrealized Losses	(152)	(1)
Estimated Fair Value	222,024	184,167
U.S. Government Agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	64,498	54,983
Gross Unrealized Gains	125	406
Gross Unrealized Losses	(29)	0
Estimated Fair Value	64,594	55,389
State and municipal
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	150,995	123,075
Gross Unrealized Gains	6,194	5,244
Gross Unrealized Losses	(123)	(1)
Estimated Fair Value	157,066	128,318
Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	321	407
Gross Unrealized Gains	43	53
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 364	$ 460