Operating Segments and Reconciliation of Information to Condensed Consolidated Financial Statements (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Interest income	$ 36,279	$ 34,615
Interest expense	4,953	5,910
Provision for loan and lease losses	78	664	3,649
Noninterest income	12,419	10,974
Noninterest expenses	27,823	26,683
Income before income taxes	15,844	12,332
Income tax expense	5,286	3,856
Net income	10,558	8,476
Assets	3,932,026	3,668,273	3,955,206
Community Banking
Segment Reporting Information [Line Items]
Interest income	36,279	34,615
Interest expense	4,958	5,914
Provision for loan and lease losses	78	664
Noninterest income	9,706	8,494
Noninterest expenses	26,044	24,867
Income before income taxes	14,905	11,664
Income tax expense	4,915	3,594
Net income	9,990	8,070
Assets	3,976,665	3,680,127
Insurance
Segment Reporting Information [Line Items]
Interest income	2	1
Interest expense	0	0
Provision for loan and lease losses	0	0
Noninterest income	1,407	1,286
Noninterest expenses	1,126	1,214
Income before income taxes	283	73
Income tax expense	115	30
Net income	168	43
Assets	13,659	13,051
Investment Management
Segment Reporting Information [Line Items]
Interest income	3	3
Interest expense	0	0
Provision for loan and lease losses	0	0
Noninterest income	1,509	1,397
Noninterest expenses	856	805
Income before income taxes	656	595
Income tax expense	256	232
Net income	400	363
Assets	17,889	15,145
Intersegment Elimination
Segment Reporting Information [Line Items]
Interest income	(5)	(4)
Interest expense	(5)	(4)
Provision for loan and lease losses	0	0
Noninterest income	(203)	(203)
Noninterest expenses	(203)	(203)
Income before income taxes	0	0
Income tax expense	0	0
Net income	0	0
Assets	$ (76,187)	$ (40,050)