Reserve for Loan and Lease Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013 item	Jun. 30, 2012	Jun. 30, 2013 item	Jun. 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	$ 84,011	$ 82,394	$ 83,311	$ 81,644
Charge-offs	491	1,673	1,453	4,440
Recoveries	877	523	1,782	1,786
Net charge-offs (recoveries)	(386)	1,150	(329)	2,654
Provision (recovery of provision)	1,293	2,055	2,050	4,309
Balance at the end of the period	85,690	83,299	85,690	83,299
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	759	2,280	759	2,280
Ending balance, collectively evaluated for impairment	84,931	81,019	84,931	81,019
Total reserve for loan and lease losses	85,690	83,299	85,690	83,299
Recorded investment in loans
Ending balance, individually evaluated for impairment	38,673	47,347	38,673	47,347
Ending balance, collectively evaluated for impairment	3,454,712	3,223,245	3,454,712	3,223,245
Total loans and leases	3,493,385	3,270,592	3,493,385	3,270,592	3,327,553
Commercial and agricultural loans
Changes in reserve for loan and lease losses
Balance at the beginning of the period	13,227	12,525	12,326	13,091
Charge-offs	129	126	414	272
Recoveries	132	68	241	164
Net charge-offs (recoveries)	(3)	58	173	108
Provision (recovery of provision)	(1,028)	610	49	94
Balance at the end of the period	12,202	13,077	12,202	13,077
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	222	1,073	222	1,073
Ending balance, collectively evaluated for impairment	11,980	12,004	11,980	12,004
Total reserve for loan and lease losses	12,202	13,077	12,202	13,077
Recorded investment in loans
Ending balance, individually evaluated for impairment	12,208	8,946	12,208	8,946
Ending balance, collectively evaluated for impairment	648,172	547,040	648,172	547,040
Total loans and leases	660,380	555,986	660,380	555,986	639,069
Auto, light truck and environmental equipment
Changes in reserve for loan and lease losses
Balance at the beginning of the period	9,330	9,769	9,584	8,469
Charge-offs		867	1	2,900
Recoveries	15	75	138	858
Net charge-offs (recoveries)	(15)	792	(137)	2,042
Provision (recovery of provision)	2,544	1,323	2,168	3,873
Balance at the end of the period	11,889	10,300	11,889	10,300
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment		500		500
Ending balance, collectively evaluated for impairment	11,889	9,800	11,889	9,800
Total reserve for loan and lease losses	11,889	10,300	11,889	10,300
Recorded investment in loans
Ending balance, individually evaluated for impairment	142	2,643	142	2,643
Ending balance, collectively evaluated for impairment	510,420	505,850	510,420	505,850
Total loans and leases	510,562	508,493	510,562	508,493	438,147
Medium and heavy duty truck
Changes in reserve for loan and lease losses
Balance at the beginning of the period	2,839	3,667	3,001	3,742
Recoveries	15	1	29	22
Net charge-offs (recoveries)	(15)	(1)	(29)	(22)
Provision (recovery of provision)	324	(50)	148	(146)
Balance at the end of the period	3,178	3,618	3,178	3,618
Reserve for loan and lease losses
Ending balance, collectively evaluated for impairment	3,178	3,618	3,178	3,618
Total reserve for loan and lease losses	3,178	3,618	3,178	3,618
Recorded investment in loans
Ending balance, individually evaluated for impairment	711	787	711	787
Ending balance, collectively evaluated for impairment	177,883	171,518	177,883	171,518
Total loans and leases	178,594	172,305	178,594	172,305	172,002
Aircraft financing
Changes in reserve for loan and lease losses
Balance at the beginning of the period	33,832	28,598	34,205	28,626
Charge-offs			31	139
Recoveries	506	196	684	321
Net charge-offs (recoveries)	(506)	(196)	(653)	(182)
Provision (recovery of provision)	(1,015)	1,077	(1,535)	1,063
Balance at the end of the period	33,323	29,871	33,323	29,871
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	537	688	537	688
Ending balance, collectively evaluated for impairment	32,786	29,183	32,786	29,183
Total reserve for loan and lease losses	33,323	29,871	33,323	29,871
Recorded investment in loans
Ending balance, individually evaluated for impairment	4,424	6,308	4,424	6,308
Ending balance, collectively evaluated for impairment	673,086	655,876	673,086	655,876
Total loans and leases	677,510	662,184	677,510	662,184	696,479
Construction equipment financing
Changes in reserve for loan and lease losses
Balance at the beginning of the period	5,725	6,851	5,390	6,295
Charge-offs	5		88	119
Recoveries	39	50	74	84
Net charge-offs (recoveries)	(34)	(50)	14	35
Provision (recovery of provision)	306	(571)	689	70
Balance at the end of the period	6,065	6,330	6,065	6,330
Reserve for loan and lease losses
Ending balance, collectively evaluated for impairment	6,065	6,330	6,065	6,330
Total reserve for loan and lease losses	6,065	6,330	6,065	6,330
Recorded investment in loans
Ending balance, individually evaluated for impairment	3,895	5,022	3,895	5,022
Ending balance, collectively evaluated for impairment	307,240	275,693	307,240	275,693
Total loans and leases	311,135	280,715	311,135	280,715	278,974
Commercial real estate
Changes in reserve for loan and lease losses
Balance at the beginning of the period	13,921	16,326	13,778	16,772
Charge-offs	102	109	164	142
Recoveries	81	45	410	79
Net charge-offs (recoveries)	21	64	(246)	63
Provision (recovery of provision)	(371)	(1,090)	(495)	(1,537)
Balance at the end of the period	13,529	15,172	13,529	15,172
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment		19		19
Ending balance, collectively evaluated for impairment	13,529	15,153	13,529	15,153
Total reserve for loan and lease losses	13,529	15,172	13,529	15,172
Recorded investment in loans
Ending balance, individually evaluated for impairment	17,293	23,535	17,293	23,535
Ending balance, collectively evaluated for impairment	559,517	520,157	559,517	520,157
Total loans and leases	576,810	543,692	576,810	543,692	554,968
Residential real estate
Changes in reserve for loan and lease losses
Balance at the beginning of the period	3,706	3,378	3,652	3,362
Charge-offs	77	32	154	73
Recoveries	5	2	11	34
Net charge-offs (recoveries)	72	30	143	39
Provision (recovery of provision)	261	173	386	198
Balance at the end of the period	3,895	3,521	3,895	3,521
Reserve for loan and lease losses
Ending balance, collectively evaluated for impairment	3,895	3,521	3,895	3,521
Total reserve for loan and lease losses	3,895	3,521	3,895	3,521
Recorded investment in loans
Ending balance, individually evaluated for impairment		106		106
Ending balance, collectively evaluated for impairment	454,983	441,481	454,983	441,481
Total loans and leases	454,983	441,587	454,983	441,587	438,641
Consumer loans
Changes in reserve for loan and lease losses
Balance at the beginning of the period	1,431	1,280	1,375	1,287
Charge-offs	178	539	601	795
Recoveries	84	86	195	224
Net charge-offs (recoveries)	94	453	406	571
Provision (recovery of provision)	272	583	640	694
Balance at the end of the period	1,609	1,410	1,609	1,410
Reserve for loan and lease losses
Ending balance, collectively evaluated for impairment	1,609	1,410	1,609	1,410
Total reserve for loan and lease losses	1,609	1,410	1,609	1,410
Recorded investment in loans
Ending balance, collectively evaluated for impairment	123,411	105,630	123,411	105,630
Total loans and leases	$ 123,411	$ 105,630	$ 123,411	$ 105,630	$ 109,273