Loan and Lease Finance Receivables and Allowance for Credit Losses - Summary of Internal Risk Grouping by Loan Class (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Commercial & industrial	$ 558,255	$ 573,571
Residential real estate (SFR 1-4)	163,343	160,703
Dairy and livestock	278,502	327,579
Agribusiness	11,240	14,732
Municipal lease finance receivables	109,727	105,767
Consumer	50,083	53,894
Auto and equipment leases	12,422	12,716
Covered loans	199,602	220,559
Total gross loans	3,396,603	3,479,797
Non-Covered Loans [Member]
Commercial & industrial	532,941	547,422
Construction - speculative	35,627	37,894
Construction - non-speculative	20,076	21,827
Commercial real estate - owner occupied	691,147	703,967
Commercial real estate - non-owner occupied	1,305,051	1,286,140
Residential real estate (SFR 1-4)	162,022	159,288
Dairy and livestock	278,502	327,579
Agribusiness	5,370	9,081
Municipal lease finance receivables	109,727	105,767
Consumer	44,116	47,557
Auto and equipment leases	12,422	12,716
Total non-covered loans	3,197,001	3,259,238
Pass [Member]
Covered loans	47,139	52,637
Total gross loans	1,992,037	2,005,654
Pass [Member] | Non-Covered Loans [Member]
Commercial & industrial	338,974	347,275
Construction - speculative	3,756	1,417
Construction - non-speculative	6,540	9,841
Commercial real estate - owner occupied	374,300	382,111
Commercial real estate - non-owner occupied	926,150	888,777
Residential real estate (SFR 1-4)	131,708	129,730
Dairy and livestock	48,015	67,144
Agribusiness	3,630	4,969
Municipal lease finance receivables	65,673	72,432
Consumer	37,625	40,650
Auto and equipment leases	8,527	8,671
Total non-covered loans	1,944,898	1,953,017
Watch List [Member]
Covered loans	69,482	72,803
Total gross loans	655,658	729,918
Watch List [Member] | Non-Covered Loans [Member]
Commercial & industrial	119,560	131,186
Construction - non-speculative	4,317	2,767
Commercial real estate - owner occupied	142,436	159,653
Commercial real estate - non-owner occupied	215,660	214,901
Residential real estate (SFR 1-4)	11,158	10,215
Dairy and livestock	64,557	108,087
Agribusiness	1,050	3,306
Municipal lease finance receivables	21,249	20,237
Consumer	3,206	3,538
Auto and equipment leases	2,983	3,225
Total non-covered loans	586,176	657,115
Special Mention [Member]
Covered loans	22,465	31,689
Total gross loans	368,858	366,787
Special Mention [Member] | Non-Covered Loans [Member]
Commercial & industrial	48,081	44,466
Construction - speculative	13,599	15,163
Commercial real estate - owner occupied	88,461	78,087
Commercial real estate - non-owner occupied	94,016	105,121
Residential real estate (SFR 1-4)	4,273	3,107
Dairy and livestock	80,048	74,510
Agribusiness	690	806
Municipal lease finance receivables	14,535	11,124
Consumer	1,914	1,976
Auto and equipment leases	776	738
Total non-covered loans	346,393	335,098
Substandard [Member]
Covered loans	60,516	63,354
Total gross loans	374,591	375,597
Substandard [Member] | Non-Covered Loans [Member]
Commercial & industrial	24,931	22,901
Construction - speculative	18,272	21,314
Construction - non-speculative	9,219	9,219
Commercial real estate - owner occupied	85,950	84,116
Commercial real estate - non-owner occupied	69,225	77,341
Residential real estate (SFR 1-4)	14,883	16,236
Dairy and livestock	81,822	77,721
Municipal lease finance receivables	8,270	1,974
Consumer	1,367	1,339
Auto and equipment leases	136	82
Total non-covered loans	314,075	312,243
Doubtful & Loss [Member]
Covered loans		76
Total gross loans	5,459	1,841
Doubtful & Loss [Member] | Non-Covered Loans [Member]
Commercial & industrial	1,395	1,594
Dairy and livestock	4,060	117
Consumer	4	54
Total non-covered loans	$ 5,459	$ 1,765