Operating Segments and Reconciliation of Information to Condensed Consolidated Financial Statements (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Segment Reporting Information [Line Items]
Interest income	$ 35,779	$ 35,558	$ 72,058	$ 70,173
Interest expense	4,847	5,749	9,800	11,659
Provision (credit) for loan and lease losses	(2,876)	1,585	(2,798)	2,249
Non-interest income	12,215	11,493	24,634	22,467
Non-interest expenses	27,508	28,858	55,331	55,541
Income before income taxes	18,515	10,859	34,359	23,191
Income tax expense	6,353	3,652	11,639	7,508
Net income	12,162	7,207	22,720	15,683
Assets	4,072,617	3,855,177	4,072,617	3,855,177	3,955,206
Community Banking
Segment Reporting Information [Line Items]
Interest income	35,779	35,558	72,058	70,173
Interest expense	4,854	5,753	9,812	11,667
Provision (credit) for loan and lease losses	(2,876)	1,585	(2,798)	2,249
Non-interest income	9,589	9,306	19,295	17,800
Non-interest expenses	25,910	27,365	51,954	52,232
Income before income taxes	17,480	10,161	32,385	21,825
Income tax expense	5,946	3,379	10,861	6,973
Net income	11,534	6,782	21,524	14,852
Assets	4,087,948	3,870,262	4,087,948	3,870,262
Insurance
Segment Reporting Information [Line Items]
Interest income	3	2	5	3
Interest expense	0	0	0	0
Provision (credit) for loan and lease losses	0	0	0	0
Non-interest income	1,290	1,026	2,697	2,312
Non-interest expenses	1,009	954	2,135	2,168
Income before income taxes	284	74	567	147
Income tax expense	115	30	230	60
Net income	169	44	337	87
Assets	13,856	13,180	13,856	13,180
Investment Management
Segment Reporting Information [Line Items]
Interest income	4	2	7	5
Interest expense	0	0	0	0
Provision (credit) for loan and lease losses	0	0	0	0
Non-interest income	1,540	1,364	3,049	2,761
Non-interest expenses	793	742	1,649	1,547
Income before income taxes	751	624	1,407	1,219
Income tax expense	292	243	548	475
Net income	459	381	859	744
Assets	17,188	15,537	17,188	15,537
Intersegment Elimination
Segment Reporting Information [Line Items]
Interest income	(7)	(4)	(12)	(8)
Interest expense	(7)	(4)	(12)	(8)
Provision (credit) for loan and lease losses	0	0	0	0
Non-interest income	(204)	(203)	(407)	(406)
Non-interest expenses	(204)	(203)	(407)	(406)
Income before income taxes	0	0	0	0
Income tax expense	0	0	0	0
Net income	0	0	0	0
Assets	$ (46,375)	$ (43,802)	$ (46,375)	$ (43,802)