Reserve for Loan and Lease Losses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014 item	Mar. 31, 2013	Dec. 31, 2013
Reserve for loan and lease losses
Number of classes existing in loan and lease portfolio	8
Changes in reserve for loan and lease losses
Balance at the beginning of the period	$ 83,505	$ 83,311
Charge-offs	303	962
Recoveries	1,004	905
Net charge-offs (recoveries)	(701)	57
Provision (recovery of provision)	804	757
Balance at the end of the period	85,010	84,011
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	226	2,073
Ending balance, collectively evaluated for impairment	84,784	81,938
Total reserve for loan and lease losses	85,010	84,011
Recorded investment in loans
Ending balance, individually evaluated for impairment	29,854	46,655
Ending balance, collectively evaluated for impairment	3,547,714	3,326,563
Total loans and leases	3,577,568	3,373,218	3,549,324
Commercial and agricultural loans
Changes in reserve for loan and lease losses
Balance at the beginning of the period	11,515	12,326
Charge-offs	15	285
Recoveries	379	109
Net charge-offs (recoveries)	(364)	176
Provision (recovery of provision)	(60)	1,077
Balance at the end of the period	11,819	13,227
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	0	434
Ending balance, collectively evaluated for impairment	11,819	12,793
Total reserve for loan and lease losses	11,819	13,227
Recorded investment in loans
Ending balance, individually evaluated for impairment	11,146	7,072
Ending balance, collectively evaluated for impairment	687,100	640,589
Total loans and leases	698,246	647,661	679,492
Auto and light truck
Changes in reserve for loan and lease losses
Balance at the beginning of the period	9,657	8,864
Charge-offs	11	1
Recoveries	234	121
Net charge-offs (recoveries)	(223)	(120)
Provision (recovery of provision)	(224)	(288)
Balance at the end of the period	9,656	8,696
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	0	0
Ending balance, collectively evaluated for impairment	9,656	8,696
Total reserve for loan and lease losses	9,656	8,696
Recorded investment in loans
Ending balance, individually evaluated for impairment	266	0
Ending balance, collectively evaluated for impairment	388,399	412,569
Total loans and leases	388,665	412,569	391,649
Medium and heavy duty truck
Changes in reserve for loan and lease losses
Balance at the beginning of the period	4,212	3,721
Charge-offs	0	0
Recoveries	37	16
Net charge-offs (recoveries)	(37)	(16)
Provision (recovery of provision)	162	(264)
Balance at the end of the period	4,411	3,473
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	0	0
Ending balance, collectively evaluated for impairment	4,411	3,473
Total reserve for loan and lease losses	4,411	3,473
Recorded investment in loans
Ending balance, individually evaluated for impairment	0	464
Ending balance, collectively evaluated for impairment	237,906	203,203
Total loans and leases	237,906	203,667	237,854
Aircraft financing
Changes in reserve for loan and lease losses
Balance at the beginning of the period	34,037	34,205
Charge-offs	0	31
Recoveries	57	178
Net charge-offs (recoveries)	(57)	(147)
Provision (recovery of provision)	415	(520)
Balance at the end of the period	34,509	33,832
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	57	1,639
Ending balance, collectively evaluated for impairment	34,452	32,193
Total reserve for loan and lease losses	34,509	33,832
Recorded investment in loans
Ending balance, individually evaluated for impairment	4,147	14,762
Ending balance, collectively evaluated for impairment	726,656	684,479
Total loans and leases	730,803	699,241	738,133
Construction equipment financing
Changes in reserve for loan and lease losses
Balance at the beginning of the period	5,972	5,390
Charge-offs	2	83
Recoveries	166	35
Net charge-offs (recoveries)	(164)	48
Provision (recovery of provision)	178	383
Balance at the end of the period	6,314	5,725
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	9	0
Ending balance, collectively evaluated for impairment	6,305	5,725
Total reserve for loan and lease losses	6,314	5,725
Recorded investment in loans
Ending balance, individually evaluated for impairment	1,129	4,154
Ending balance, collectively evaluated for impairment	351,667	281,762
Total loans and leases	352,796	285,916	333,088
Commercial real estate
Changes in reserve for loan and lease losses
Balance at the beginning of the period	12,406	13,778
Charge-offs	1	62
Recoveries	39	329
Net charge-offs (recoveries)	(38)	(267)
Provision (recovery of provision)	165	(124)
Balance at the end of the period	12,609	13,921
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	0	0
Ending balance, collectively evaluated for impairment	12,609	13,921
Total reserve for loan and lease losses	12,609	13,921
Recorded investment in loans
Ending balance, individually evaluated for impairment	12,789	20,203
Ending balance, collectively evaluated for impairment	575,840	546,152
Total loans and leases	588,629	566,355	583,997
Residential real estate
Changes in reserve for loan and lease losses
Balance at the beginning of the period	4,093	3,652
Charge-offs	16	77
Recoveries	5	6
Net charge-offs (recoveries)	11	71
Provision (recovery of provision)	(20)	125
Balance at the end of the period	4,062	3,706
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	160	0
Ending balance, collectively evaluated for impairment	3,902	3,706
Total reserve for loan and lease losses	4,062	3,706
Recorded investment in loans
Ending balance, individually evaluated for impairment	377	0
Ending balance, collectively evaluated for impairment	455,301	445,160
Total loans and leases	455,678	445,160	460,981
Consumer loans
Changes in reserve for loan and lease losses
Balance at the beginning of the period	1,613	1,375
Charge-offs	258	423
Recoveries	87	111
Net charge-offs (recoveries)	171	312
Provision (recovery of provision)	188	368
Balance at the end of the period	1,630	1,431
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment	0	0
Ending balance, collectively evaluated for impairment	1,630	1,431
Total reserve for loan and lease losses	1,630	1,431
Recorded investment in loans
Ending balance, individually evaluated for impairment	0	0
Ending balance, collectively evaluated for impairment	124,845	112,649
Total loans and leases	$ 124,845	$ 112,649	$ 124,130