Amortized Cost and Estimated Fair Values of Investments Held-To-Maturity (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 225,994	$ 215,814
Gross Unrealized Gains	2,254	6,362
Gross Unrealized Losses	(8,194)	(152)
Investments held-to-maturity, fair value	220,054	222,024
U.S. Government Agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	64,503	64,498
Gross Unrealized Gains	0	125
Gross Unrealized Losses	(3,381)	(29)
Investments held-to-maturity, fair value	61,122	64,594
State and municipal
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	161,231	150,995
Gross Unrealized Gains	2,219	6,194
Gross Unrealized Losses	(4,813)	(123)
Investments held-to-maturity, fair value	158,637	157,066
Mortgage-Backed
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	260	321
Gross Unrealized Gains	35	43
Gross Unrealized Losses	0	0
Investments held-to-maturity, fair value	$ 295	$ 364