Reserve for Loan and Lease Losses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Changes in reserve for loan and lease losses
Balance at the beginning of the period	$ 88,543	$ 88,112	$ 85,068
Charge-offs	6,529	7,941	4,710
Recoveries	3,889	2,539	5,594
Net charge-offs (recoveries)	2,640	5,402	(884)
Provision (recovery of provision)	8,980	5,833	2,160
Balance at the end of the period	94,883	88,543	88,112
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment				$ 666	$ 1,878
Ending balance, collectively evaluated for impairment				94,217	86,665
Total reserve for loan and lease losses	88,543	88,112	85,068	94,883	88,543
Recorded investment in loans
Ending balance, individually evaluated for impairment				17,117	17,028
Ending balance, collectively evaluated for impairment				4,510,561	4,171,043
Total loans and leases				4,527,678	4,188,071
Commercial and agricultural
Changes in reserve for loan and lease losses
Balance at the beginning of the period	14,668	15,456	11,760
Charge-offs	2,415	547	3,489
Recoveries	984	509	851
Net charge-offs (recoveries)	1,431	38	2,638
Provision (recovery of provision)	2,991	(750)	6,334
Balance at the end of the period	16,228	14,668	15,456
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment				0	297
Ending balance, collectively evaluated for impairment				16,228	14,371
Total reserve for loan and lease losses	14,668	15,456	11,760	16,228	14,668
Recorded investment in loans
Ending balance, individually evaluated for impairment				2,439	3,590
Ending balance, collectively evaluated for impairment				927,558	808,674
Total loans and leases				929,997	812,264
Auto and light truck
Changes in reserve for loan and lease losses
Balance at the beginning of the period	8,064	9,269	10,326
Charge-offs	774	4	24
Recoveries	1,153	253	380
Net charge-offs (recoveries)	(379)	(249)	(356)
Provision (recovery of provision)	1,660	(1,454)	(1,413)
Balance at the end of the period	10,103	8,064	9,269
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment				243	0
Ending balance, collectively evaluated for impairment				9,860	8,064
Total reserve for loan and lease losses	8,064	9,269	10,326	10,103	8,064
Recorded investment in loans
Ending balance, individually evaluated for impairment				7,780	115
Ending balance, collectively evaluated for impairment				489,036	411,649
Total loans and leases				496,816	411,764
Medium and heavy duty truck
Changes in reserve for loan and lease losses
Balance at the beginning of the period	4,740	4,699	4,500
Charge-offs	0	0	0
Recoveries	0	10	28
Net charge-offs (recoveries)	0	(10)	(28)
Provision (recovery of provision)	104	31	171
Balance at the end of the period	4,844	4,740	4,699
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment				0	0
Ending balance, collectively evaluated for impairment				4,844	4,740
Total reserve for loan and lease losses	4,740	4,699	4,500	4,844	4,740
Recorded investment in loans
Ending balance, individually evaluated for impairment				371	0
Ending balance, collectively evaluated for impairment				296,564	294,790
Total loans and leases				296,935	294,790
Aircraft
Changes in reserve for loan and lease losses
Balance at the beginning of the period	34,352	32,373	32,234
Charge-offs	1,872	6,123	244
Recoveries	227	528	802
Net charge-offs (recoveries)	1,645	5,595	(558)
Provision (recovery of provision)	1,912	7,574	(419)
Balance at the end of the period	34,619	34,352	32,373
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment				0	1,076
Ending balance, collectively evaluated for impairment				34,619	33,276
Total reserve for loan and lease losses	34,352	32,373	32,234	34,619	34,352
Recorded investment in loans
Ending balance, individually evaluated for impairment				1,901	6,110
Ending balance, collectively evaluated for impairment				842,756	796,304
Total loans and leases				844,657	802,414
Construction equipment
Changes in reserve for loan and lease losses
Balance at the beginning of the period	8,207	7,592	7,008
Charge-offs	164	128	0
Recoveries	298	461	434
Net charge-offs (recoveries)	(134)	(333)	(434)
Provision (recovery of provision)	1,002	282	150
Balance at the end of the period	9,343	8,207	7,592
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment				108	35
Ending balance, collectively evaluated for impairment				9,235	8,172
Total reserve for loan and lease losses	8,207	7,592	7,008	9,343	8,207
Recorded investment in loans
Ending balance, individually evaluated for impairment				928	1,167
Ending balance, collectively evaluated for impairment				562,509	494,758
Total loans and leases				563,437	495,925
Commercial real estate
Changes in reserve for loan and lease losses
Balance at the beginning of the period	13,677	13,762	13,270
Charge-offs	344	32	0
Recoveries	851	469	2,807
Net charge-offs (recoveries)	(507)	(437)	(2,807)
Provision (recovery of provision)	608	(522)	(2,315)
Balance at the end of the period	14,792	13,677	13,762
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment				181	322
Ending balance, collectively evaluated for impairment				14,611	13,355
Total reserve for loan and lease losses	13,677	13,762	13,270	14,792	13,677
Recorded investment in loans
Ending balance, individually evaluated for impairment				3,346	5,372
Ending balance, collectively evaluated for impairment				738,222	713,798
Total loans and leases				741,568	719,170
Residential real estate and home equity
Changes in reserve for loan and lease losses
Balance at the beginning of the period	3,550	3,662	4,504
Charge-offs	124	219	295
Recoveries	109	31	34
Net charge-offs (recoveries)	15	188	261
Provision (recovery of provision)	131	76	(581)
Balance at the end of the period	3,666	3,550	3,662
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment				134	148
Ending balance, collectively evaluated for impairment				3,532	3,402
Total reserve for loan and lease losses	3,550	3,662	4,504	3,666	3,550
Recorded investment in loans
Ending balance, individually evaluated for impairment				352	674
Ending balance, collectively evaluated for impairment				525,770	521,257
Total loans and leases				526,122	521,931
Consumer
Changes in reserve for loan and lease losses
Balance at the beginning of the period	1,285	1,299	1,466
Charge-offs	836	888	658
Recoveries	267	278	258
Net charge-offs (recoveries)	569	610	400
Provision (recovery of provision)	572	596	233
Balance at the end of the period	1,288	1,285	1,299
Reserve for loan and lease losses
Ending balance, individually evaluated for impairment				0	0
Ending balance, collectively evaluated for impairment				1,288	1,285
Total reserve for loan and lease losses	$ 1,285	$ 1,299	$ 1,466	1,288	1,285
Recorded investment in loans
Ending balance, individually evaluated for impairment				0	0
Ending balance, collectively evaluated for impairment				128,146	129,813
Total loans and leases				$ 128,146	$ 129,813