Amortized Cost and Estimated Fair Values of Investments Held-To-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 164,846	$ 178,465
Gross Unrealized Gains	4,967	5,703
Gross Unrealized Losses	(417)	(1)
Estimated Fair Value	169,396	184,167
U.S. Government Agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37,576	54,983
Gross Unrealized Gains	37	406
Gross Unrealized Losses	(140)
Estimated Fair Value	37,473	55,389
US States and Political Subdivisions Debt Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	126,904	123,075
Gross Unrealized Gains	4,883	5,244
Gross Unrealized Losses	(277)	(1)
Estimated Fair Value	131,510	128,318
Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	366	407
Gross Unrealized Gains	47	53
Estimated Fair Value	$ 413	$ 460