Amortized Cost and Estimated Fair Values of Investments Held-To-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 206,613	$ 178,465
Gross Unrealized Gains	6,682	5,703
Gross Unrealized Losses	(60)	(1)
Estimated Fair Value	213,235	184,167
U.S. Government Agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	59,500	54,983
Gross Unrealized Gains	211	406
Estimated Fair Value	59,711	55,389
US States and Political Subdivisions Debt Securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	146,769	123,075
Gross Unrealized Gains	6,426	5,244
Gross Unrealized Losses	(60)	(1)
Estimated Fair Value	153,135	128,318
Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	344	407
Gross Unrealized Gains	45	53
Estimated Fair Value	$ 389	$ 460