Amortized Cost and Estimated Fair Values of Investments Held-To-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 211,376	$ 215,814
Gross Unrealized Gains	5,356	6,362
Gross Unrealized Losses	(312)	(152)
Estimated Fair Value	216,420	222,024
U.S. Government Agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	64,500	64,498
Gross Unrealized Gains	127	125
Gross Unrealized Losses	(11)	(29)
Estimated Fair Value	64,616	64,594
State and municipal
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	146,583	150,995
Gross Unrealized Gains	5,189	6,194
Gross Unrealized Losses	(301)	(123)
Estimated Fair Value	151,471	157,066
Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	293	321
Gross Unrealized Gains	40	43
Gross Unrealized Losses		0
Estimated Fair Value	$ 333	$ 364