Reserve for Loan and Lease Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013 item	Sep. 30, 2012	Sep. 30, 2013 item	Sep. 30, 2012	Dec. 31, 2012
Reserve for loan and lease losses
Number of classes existing in loan and lease portfolio	8		8
Changes in reserve for loan and lease losses
Balance at the beginning of the period	$ 85,690	$ 83,299	$ 83,311	$ 81,644
Charge-offs	1,671	1,488	3,124	5,928
Recoveries	907	1,038	2,689	2,824
Net charge-offs (recoveries)	764	450	435	3,104
Provision (recovery of provision)	(419)	650	1,631	4,959
Balance at the end of the period	84,507	83,499	84,507	83,499
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	1,540	2,313	1,540	2,313
Ending balance, collectively evaluated for impairment	82,967	81,186	82,967	81,186
Total reserve for loan and lease losses	84,507	83,499	84,507	83,499
Recorded investment in loans
Ending balance, individually evaluated for impairment	40,239	44,419	40,239	44,419
Ending balance, collectively evaluated for impairment	3,427,879	3,223,994	3,427,879	3,223,994
Total loans and leases	3,468,118	3,268,413	3,468,118	3,268,413	3,327,553
Commercial and agricultural loans
Changes in reserve for loan and lease losses
Balance at the beginning of the period	12,202	13,077	12,326	13,091
Charge-offs	49	214	463	486
Recoveries	114	60	355	224
Net charge-offs (recoveries)	(65)	154	108	262
Provision (recovery of provision)	(255)	433	(206)	527
Balance at the end of the period	12,012	13,356	12,012	13,356
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	49	878	49	878
Ending balance, collectively evaluated for impairment	11,963	12,478	11,963	12,478
Total reserve for loan and lease losses	12,012	13,356	12,012	13,356
Recorded investment in loans
Ending balance, individually evaluated for impairment	11,659	9,148	11,659	9,148
Ending balance, collectively evaluated for impairment	640,521	575,848	640,521	575,848
Total loans and leases	652,180	584,996	652,180	584,996	639,069
Auto, light truck and environmental equipment
Changes in reserve for loan and lease losses
Balance at the beginning of the period	11,889	10,300	9,584	8,469
Charge-offs	50	210	51	3,110
Recoveries	98	356	236	1,214
Net charge-offs (recoveries)	(48)	(146)	(185)	1,896
Provision (recovery of provision)	(1,588)	(496)	580	3,377
Balance at the end of the period	10,349	9,950	10,349	9,950
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment		500		500
Ending balance, collectively evaluated for impairment	10,349	9,450	10,349	9,450
Total reserve for loan and lease losses	10,349	9,950	10,349	9,950
Recorded investment in loans
Ending balance, individually evaluated for impairment		2,233		2,233
Ending balance, collectively evaluated for impairment	452,405	454,432	452,405	454,432
Total loans and leases	452,405	456,665	452,405	456,665	438,147
Medium and heavy duty truck
Changes in reserve for loan and lease losses
Balance at the beginning of the period	3,178	3,618	3,001	3,742
Recoveries	308	125	337	147
Net charge-offs (recoveries)	(308)	(125)	(337)	(147)
Provision (recovery of provision)	(83)	(377)	65	(523)
Balance at the end of the period	3,403	3,366	3,403	3,366
Reserve for loan and lease losses
Ending balance, collectively evaluated for impairment	3,403	3,366	3,403	3,366
Total reserve for loan and lease losses	3,403	3,366	3,403	3,366
Recorded investment in loans
Ending balance, individually evaluated for impairment	594	322	594	322
Ending balance, collectively evaluated for impairment	192,380	167,387	192,380	167,387
Total loans and leases	192,974	167,709	192,974	167,709	172,002
Aircraft financing
Changes in reserve for loan and lease losses
Balance at the beginning of the period	33,323	29,871	34,205	28,626
Charge-offs	1,277	461	1,308	600
Recoveries	98	163	782	484
Net charge-offs (recoveries)	1,179	298	526	116
Provision (recovery of provision)	1,670	1,427	135	2,490
Balance at the end of the period	33,814	31,000	33,814	31,000
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	1,491	886	1,491	886
Ending balance, collectively evaluated for impairment	32,323	30,114	32,323	30,114
Total reserve for loan and lease losses	33,814	31,000	33,814	31,000
Recorded investment in loans
Ending balance, individually evaluated for impairment	11,433	5,734	11,433	5,734
Ending balance, collectively evaluated for impairment	692,639	680,066	692,639	680,066
Total loans and leases	704,072	685,800	704,072	685,800	696,479
Construction equipment financing
Changes in reserve for loan and lease losses
Balance at the beginning of the period	6,065	6,330	5,390	6,295
Charge-offs		1	88	120
Recoveries	64	149	138	233
Net charge-offs (recoveries)	(64)	(148)	(50)	(113)
Provision (recovery of provision)	448	(585)	1,137	(515)
Balance at the end of the period	6,577	5,893	6,577	5,893
Reserve for loan and lease losses
Ending balance, collectively evaluated for impairment	6,577	5,893	6,577	5,893
Total reserve for loan and lease losses	6,577	5,893	6,577	5,893
Recorded investment in loans
Ending balance, individually evaluated for impairment	1,214	4,355	1,214	4,355
Ending balance, collectively evaluated for impairment	314,132	271,915	314,132	271,915
Total loans and leases	315,346	276,270	315,346	276,270	278,974
Commercial real estate
Changes in reserve for loan and lease losses
Balance at the beginning of the period	13,529	15,172	13,778	16,772
Charge-offs		132	164	274
Recoveries	150	91	560	170
Net charge-offs (recoveries)	(150)	41	(396)	104
Provision (recovery of provision)	(801)	(99)	(1,296)	(1,636)
Balance at the end of the period	12,878	15,032	12,878	15,032
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment		49		49
Ending balance, collectively evaluated for impairment	12,878	14,983	12,878	14,983
Total reserve for loan and lease losses	12,878	15,032	12,878	15,032
Recorded investment in loans
Ending balance, individually evaluated for impairment	15,339	22,524	15,339	22,524
Ending balance, collectively evaluated for impairment	558,940	526,397	558,940	526,397
Total loans and leases	574,279	548,921	574,279	548,921	554,968
Residential real estate
Changes in reserve for loan and lease losses
Balance at the beginning of the period	3,895	3,521	3,652	3,362
Charge-offs	76	159	230	232
Recoveries	1	4	12	38
Net charge-offs (recoveries)	75	155	218	194
Provision (recovery of provision)	70	74	456	272
Balance at the end of the period	3,890	3,440	3,890	3,440
Reserve for loan and lease losses
Ending balance, collectively evaluated for impairment	3,890	3,440	3,890	3,440
Total reserve for loan and lease losses	3,890	3,440	3,890	3,440
Recorded investment in loans
Ending balance, individually evaluated for impairment		103		103
Ending balance, collectively evaluated for impairment	455,327	436,806	455,327	436,806
Total loans and leases	455,327	436,909	455,327	436,909	438,641
Consumer loans
Changes in reserve for loan and lease losses
Balance at the beginning of the period	1,609	1,410	1,375	1,287
Charge-offs	219	311	820	1,106
Recoveries	74	90	269	314
Net charge-offs (recoveries)	145	221	551	792
Provision (recovery of provision)	120	273	760	967
Balance at the end of the period	1,584	1,462	1,584	1,462
Reserve for loan and lease losses
Ending balance, collectively evaluated for impairment	1,584	1,462	1,584	1,462
Total reserve for loan and lease losses	1,584	1,462	1,584	1,462
Recorded investment in loans
Ending balance, collectively evaluated for impairment	121,535	111,143	121,535	111,143
Total loans and leases	$ 121,535	$ 111,143	$ 121,535	$ 111,143	$ 109,273