Operating Segments and Reconciliation of Information to Condensed Consolidated Financial Statements (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Segment Reporting Information [Line Items]
Interest income	$ 37,495	$ 35,004	$ 107,668	$ 104,762
Interest expense	5,710	6,674	17,369	20,268
Provision for loan and lease losses	232	(3,520)	2,481	(854)	1,428
Noninterest income	12,242	11,336	34,709	32,130
Noninterest expenses	27,167	25,848	82,708	77,748
Income before income taxes	16,628	17,338	39,819	39,730
Income tax expense	5,638	6,081	13,146	12,886
Net income	10,990	11,257	26,673	26,844
Assets	3,887,427	3,626,043	3,887,427	3,626,043	3,711,370
Community Banking
Segment Reporting Information [Line Items]
Interest income	37,494	35,008	107,668	104,822
Interest expense	5,715	6,681	17,383	20,338
Provision for loan and lease losses	232	(3,520)	2,481	(854)
Noninterest income	9,942	9,084	27,742	25,162
Noninterest expenses	25,504	24,198	77,736	72,631
Income before income taxes	15,985	16,733	37,810	37,869
Income tax expense	5,390	5,843	12,363	12,157
Net income	10,595	10,890	25,447	25,712
Assets	3,901,588	3,655,556	3,901,588	3,655,556
Insurance Segment
Segment Reporting Information [Line Items]
Interest income	2	1	5	4
Interest expense	0	0	0	0
Provision for loan and lease losses	0	0	0	0
Noninterest income	1,076	1,136	3,388	3,526
Noninterest expenses	1,036	1,054	3,204	3,346
Income before income taxes	42	83	189	184
Income tax expense	17	35	77	76
Net income	25	48	112	108
Assets	13,058	12,715	13,058	12,715
Investment Management Segment
Segment Reporting Information [Line Items]
Interest income	4	2	9	6
Interest expense	0	0	0	0
Provision for loan and lease losses	0	0	0	0
Noninterest income	1,427	1,319	4,188	4,050
Noninterest expenses	830	799	2,377	2,379
Income before income taxes	601	522	1,820	1,677
Income tax expense	231	203	706	653
Net income	370	319	1,114	1,024
Assets	16,579	15,008	16,579	15,008
Intersegment Elimination
Segment Reporting Information [Line Items]
Interest income	(5)	(7)	(14)	(70)
Interest expense	(5)	(7)	(14)	(70)
Provision for loan and lease losses	0	0	0	0
Noninterest income	(203)	(203)	(609)	(608)
Noninterest expenses	(203)	(203)	(609)	(608)
Income before income taxes	0	0	0	0
Income tax expense	0	0	0	0
Net income	0	0	0	0
Assets	$ (43,798)	$ (57,236)	$ (43,798)	$ (57,236)