Operating Segments and Reconciliation of Information to Condensed Consolidated Financial Statements (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Interest income	$ 143,870	$ 139,470	$ 148,349
Interest expense	22,651	26,524	32,742
Provision for loan and lease losses	3,649	1,428	25,908
Noninterest income	46,956	43,500	43,782
Noninterest expenses	109,927	105,071	100,912
Income before income taxes	54,599	49,947	32,569
Income tax expense	18,045	15,845	9,049
Net income	36,554	34,102	23,520
Assets	3,955,206	3,711,370	3,519,388
Community Banking
Segment Reporting Information [Line Items]
Interest income	143,870	139,531	148,731
Interest expense	22,671	26,598	33,136
Provision for loan and lease losses	3,649	1,428	25,908
Noninterest income	42,312	33,768	33,720
Noninterest expenses	108,421	98,320	94,001
Income before income taxes	51,441	46,953	29,406
Income tax expense	16,807	14,667	7,798
Net income	34,634	32,286	21,608
Assets	3,964,926	3,742,916	3,527,889
Insurance
Segment Reporting Information [Line Items]
Interest income	8	5	7
Interest expense	0	0	0
Provision for loan and lease losses	0	0	0
Noninterest income	4,837	5,142	5,807
Noninterest expenses	4,190	4,450	4,556
Income before income taxes	655	697	1,258
Income tax expense	266	283	509
Net income	389	414	749
Assets	13,452	13,067	12,671
Investment Management
Segment Reporting Information [Line Items]
Interest income	12	8	5
Interest expense	0	0	0
Provision for loan and lease losses	0	0	0
Noninterest income	5,620	5,400	5,065
Noninterest expenses	3,129	3,111	3,165
Income before income taxes	2,503	2,297	1,905
Income tax expense	972	895	742
Net income	1,531	1,402	1,163
Assets	16,341	14,806	13,409
Intersegment Elimination
Segment Reporting Information [Line Items]
Interest income	(20)	(74)	(394)
Interest expense	(20)	(74)	(394)
Provision for loan and lease losses	0	0	0
Noninterest income	(5,813)	(810)	(810)
Noninterest expenses	(5,813)	(810)	(810)
Income before income taxes	0	0	0
Income tax expense	0	0	0
Net income	0	0	0
Assets	$ (39,513)	$ (59,419)	$ (34,581)