Loans and Lease Finance Receivables and Allowance for Loan Losses - Schedule of Recorded Investment in, and Aging of, Past Due and Nonaccrual Loans, Excluding PCI Loans by Type of Loans (Detail) (Loans, Excluding PCI Loans [Member], USD $)	Mar. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	$ 1,879,000	$ 1,543,000
60-89 Days Past Due	20,000	138,000
Total Past Due and Accruing	1,899,000	1,681,000
Nonaccrual	23,001,000	32,186,000
Current	3,585,156,000	3,665,400,000
Gross loans, excluding PCI loans	3,610,056,000	3,699,267,000
Commercial and industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	112,000	943,000
60-89 Days Past Due	0	35,000
Total Past Due and Accruing	112,000	978,000
Nonaccrual	952,000	2,308,000
Current	402,536,000	386,725,000
Gross loans, excluding PCI loans	403,600,000	390,011,000
Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	0	75,000
60-89 Days Past Due	0	0
Total Past Due and Accruing	0	75,000
Nonaccrual	2,463,000	2,481,000
Current	123,922,000	131,709,000
Gross loans, excluding PCI loans	126,385,000	134,265,000
Commercial real estate Owner occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	35,000	36,000
60-89 Days Past Due	0	86,000
Total Past Due and Accruing	35,000	122,000
Nonaccrual	2,418,000	4,072,000
Current	755,357,000	786,173,000
Gross loans, excluding PCI loans	757,810,000	790,367,000
Commercial real estate Non-owner occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	0	0
60-89 Days Past Due	0	0
Total Past Due and Accruing	0	0
Nonaccrual	14,369,000	19,246,000
Current	1,727,004,000	1,678,190,000
Gross loans, excluding PCI loans	1,741,373,000	1,697,436,000
Construction Speculative [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	0	0
60-89 Days Past Due	0	0
Total Past Due and Accruing	0	0
Nonaccrual	0	0
Current	38,146,000	36,574,000
Gross loans, excluding PCI loans	38,146,000	36,574,000
Construction Non-speculative [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	0	0
60-89 Days Past Due	0	0
Total Past Due and Accruing	0	0
Nonaccrual	0	0
Current	17,200,000	18,599,000
Gross loans, excluding PCI loans	17,200,000	18,599,000
SFR mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	1,613,000	425,000
60-89 Days Past Due	0	0
Total Past Due and Accruing	1,613,000	425,000
Nonaccrual	2,233,000	3,240,000
Current	201,286,000	201,459,000
Gross loans, excluding PCI loans	205,132,000	205,124,000
Dairy & livestock and agribusiness [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	0	0
60-89 Days Past Due	0	0
Total Past Due and Accruing	0	0
Nonaccrual	103,000	103,000
Current	173,338,000	279,070,000
Gross loans, excluding PCI loans	173,441,000	279,173,000
Municipal lease finance receivables [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	0	0
60-89 Days Past Due	0	0
Total Past Due and Accruing	0	0
Nonaccrual	0	0
Current	76,220,000	77,834,000
Gross loans, excluding PCI loans	76,220,000	77,834,000
Consumer and other loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	119,000	64,000
60-89 Days Past Due	20,000	17,000
Total Past Due and Accruing	139,000	81,000
Nonaccrual	463,000	736,000
Current	70,147,000	69,067,000
Gross loans, excluding PCI loans	$ 70,749,000	$ 69,884,000

Loans and Lease Finance Receivables and Allowance for Loan Losses - Schedule of Recorded Investment in, and Aging of, Past Due and Nonaccrual Loans, Excluding PCI Loans by Type of Loans (Detail) (Loans, Excluding PCI Loans [Member], USD $)