Loans and Lease Finance Receivables and Allowance for Loan Losses - Schedule of Held-for-Investment Loans, Excluding PCI Loans, Individually Evaluated for Impairment by Class of Loans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, Total impaired loans	$ 85,792	$ 106,909	$ 108,390
Unpaid Principal Balance, Total impaired loans	100,527	118,157	125,631
Related Allowance, Total impaired loans	1,549	3,174	2,332
Average Recorded Investment, Total impaired loans	89,031	111,263	110,479
Interest Income Recognized, Total impaired loans	3,234	3,059	2,300
Recorded Investment, With no related allowance recorded	81,905	93,932	99,925
Unpaid Principal Balance, With no related allowance recorded	95,968	104,487	116,800
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	86,433	97,003	102,609
Interest Income Recognized, With no related allowance recorded	3,234	2,850	2,227
Recorded Investment, With a related allowance recorded	3,887	12,977	8,465
Unpaid Principal Balance, With a related allowance recorded	3,624	13,670	8,831
Related Allowance, With a related allowance recorded	1,549	3,174	2,332
Average Recorded Investment, With a related allowance recorded	2,598	14,260	7,870
Interest Income Recognized, With a related allowance recorded	0	209	73
Commercial and industrial [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, With no related allowance recorded	2,391	3,055	1,936
Unpaid Principal Balance, With no related allowance recorded	3,624	3,843	2,476
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	2,487	3,248	2,131
Interest Income Recognized, With no related allowance recorded	41	66	43
Recorded Investment, With a related allowance recorded	629	293	304
Unpaid Principal Balance, With a related allowance recorded	698	301	327
Related Allowance, With a related allowance recorded	615	293	289
Average Recorded Investment, With a related allowance recorded	552	305	387
Interest Income Recognized, With a related allowance recorded	0	0	0
Small Business Administration [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, With no related allowance recorded	1,853	1,613	1,449
Unpaid Principal Balance, With no related allowance recorded	2,197	2,084	1,739
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	1,886	1,717	1,635
Interest Income Recognized, With no related allowance recorded	53	0	0
Recorded Investment, With a related allowance recorded	1,327	72	0
Unpaid Principal Balance, With a related allowance recorded	1,591	78	0
Related Allowance, With a related allowance recorded	296	72	0
Average Recorded Investment, With a related allowance recorded	714	81	0
Interest Income Recognized, With a related allowance recorded	0	0	0
Commercial real estate Owner occupied [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, With no related allowance recorded	16,961	13,041	13,478
Unpaid Principal Balance, With no related allowance recorded	18,166	14,133	14,569
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	18,027	13,463	14,459
Interest Income Recognized, With no related allowance recorded	938	548	397
Recorded Investment, With a related allowance recorded	0	0	19
Unpaid Principal Balance, With a related allowance recorded	0	0	19
Related Allowance, With a related allowance recorded	0	0	2
Average Recorded Investment, With a related allowance recorded	0	0	28
Interest Income Recognized, With a related allowance recorded	0	0	0
Commercial real estate Non-owner occupied [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, With no related allowance recorded	30,068	20,399	28,639
Unpaid Principal Balance, With no related allowance recorded	38,156	26,155	38,633
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	30,133	21,313	29,801
Interest Income Recognized, With no related allowance recorded	723	817	670
Recorded Investment, With a related allowance recorded	982	0	0
Unpaid Principal Balance, With a related allowance recorded	1,278	0	0
Related Allowance, With a related allowance recorded	154	0	0
Average Recorded Investment, With a related allowance recorded	573	0	0
Interest Income Recognized, With a related allowance recorded	0	0	0
Construction Speculative [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, With no related allowance recorded	7,651	17,617	21,314
Unpaid Principal Balance, With no related allowance recorded	7,651	18,408	21,607
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	7,651	18,043	21,650
Interest Income Recognized, With no related allowance recorded	310	310	311
Recorded Investment, With a related allowance recorded	0	0	0
Unpaid Principal Balance, With a related allowance recorded	0	0	0
Related Allowance, With a related allowance recorded	0	0	0
Average Recorded Investment, With a related allowance recorded	0	0	0
Interest Income Recognized, With a related allowance recorded	0	0	0
Construction Non-speculative [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, With no related allowance recorded	0	9,201	9,219
Unpaid Principal Balance, With no related allowance recorded	0	9,201	9,219
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	0	9,217	9,219
Interest Income Recognized, With no related allowance recorded	0	572	574
Recorded Investment, With a related allowance recorded	0	0	0
Unpaid Principal Balance, With a related allowance recorded	0	0	0
Related Allowance, With a related allowance recorded	0	0	0
Average Recorded Investment, With a related allowance recorded	0	0	0
Interest Income Recognized, With a related allowance recorded	0	0	0
SFR mortgage [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, With no related allowance recorded	6,512	10,919	11,079
Unpaid Principal Balance, With no related allowance recorded	7,493	12,516	14,342
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	6,566	10,408	11,292
Interest Income Recognized, With no related allowance recorded	110	103	54
Recorded Investment, With a related allowance recorded	467	486	3,766
Unpaid Principal Balance, With a related allowance recorded	484	489	4,071
Related Allowance, With a related allowance recorded	35	103	434
Average Recorded Investment, With a related allowance recorded	474	479	3,363
Interest Income Recognized, With a related allowance recorded	0	0	0
Dairy & livestock and agribusiness [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, With no related allowance recorded	15,796	17,702	12,406
Unpaid Principal Balance, With no related allowance recorded	17,587	17,702	13,756
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	19,060	19,205	11,834
Interest Income Recognized, With no related allowance recorded	1,057	434	173
Recorded Investment, With a related allowance recorded	0	12,110	4,303
Unpaid Principal Balance, With a related allowance recorded	0	12,783	4,340
Related Allowance, With a related allowance recorded	0	2,702	1,596
Average Recorded Investment, With a related allowance recorded	0	13,377	4,017
Interest Income Recognized, With a related allowance recorded	0	209	73
Municipal lease finance receivables [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, With no related allowance recorded	0	0	263
Unpaid Principal Balance, With no related allowance recorded	0	0	263
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	0	0	443
Interest Income Recognized, With no related allowance recorded	0	0	5
Recorded Investment, With a related allowance recorded	0	0	0
Unpaid Principal Balance, With a related allowance recorded	0	0	0
Related Allowance, With a related allowance recorded	0	0	0
Average Recorded Investment, With a related allowance recorded	0	0	0
Interest Income Recognized, With a related allowance recorded	0	0	0
Consumer and other loans [Member]
Financing Receivable, Allowance for Credit Losses [Line Items]
Recorded Investment, With no related allowance recorded	673	385	142
Unpaid Principal Balance, With no related allowance recorded	1,094	445	196
Related Allowance, With no related allowance recorded	0	0	0
Average Recorded Investment, With no related allowance recorded	623	389	145
Interest Income Recognized, With no related allowance recorded	2	0	0
Recorded Investment, With a related allowance recorded	482	16	73
Unpaid Principal Balance, With a related allowance recorded	508	19	74
Related Allowance, With a related allowance recorded	449	4	11
Average Recorded Investment, With a related allowance recorded	285	18	75
Interest Income Recognized, With a related allowance recorded	$ 0	$ 0	$ 0