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 Class of Loans (Detail) (USD $)	Dec. 31, 2014	Dec. 31, 2013
Accounts, Notes, Loans and Financing Receivable [Line Items]
Gross loans, excluding PCI loans	$ 2,780,000,000
Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	2,500,000
Loans, Excluding PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	1,543,000	2,271,000
60-89 Days Past Due	138,000	1,028,000
Total Past Due and Accruing	1,681,000	3,299,000
Nonaccrual	32,186,000	39,954,000
Current	3,665,400,000	3,351,897,000
Gross loans, excluding PCI loans	3,699,267,000	3,395,150,000
Loans, Excluding PCI Loans [Member] | Commercial and industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	943,000	900,000
60-89 Days Past Due	35,000	93,000
Total Past Due and Accruing	978,000	993,000
Nonaccrual	2,308,000	2,175,000
Current	386,725,000	373,632,000
Gross loans, excluding PCI loans	390,011,000	376,800,000
Loans, Excluding PCI Loans [Member] | Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	75,000	0
60-89 Days Past Due	0	0
Total Past Due and Accruing	75,000	0
Nonaccrual	2,481,000	1,686,000
Current	131,709,000	134,306,000
Gross loans, excluding PCI loans	134,265,000	135,992,000
Loans, Excluding PCI Loans [Member] | Commercial real estate Owner occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	36,000	220,000
60-89 Days Past Due	86,000	0
Total Past Due and Accruing	122,000	220,000
Nonaccrual	4,072,000	4,105,000
Current	786,173,000	725,626,000
Gross loans, excluding PCI loans	790,367,000	729,951,000
Loans, Excluding PCI Loans [Member] | Commercial real estate Non-owner occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	0	303,000
60-89 Days Past Due	0	0
Total Past Due and Accruing	0	303,000
Nonaccrual	19,246,000	8,305,000
Current	678,190,000	1,468,956,000
Gross loans, excluding PCI loans	1,697,436,000	1,477,564,000
Loans, Excluding PCI Loans [Member] | 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		9,966,000
Current	36,574,000	17,812,000
Gross loans, excluding PCI loans	36,574,000	27,778,000
Loans, Excluding PCI Loans [Member] | 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	18,599,000	19,331,000
Gross loans, excluding PCI loans	18,599,000	19,331,000
Loans, Excluding PCI Loans [Member] | SFR mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	425,000	773,000
60-89 Days Past Due	0	935,000
Total Past Due and Accruing	425,000	1,708,000
Nonaccrual	3,240,000	7,577,000
Current	201,459,000	179,948,000
Gross loans, excluding PCI loans	205,124,000	189,233,000
Loans, Excluding PCI Loans [Member] | 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	5,739,000
Current	279,070,000	288,553,000
Gross loans, excluding PCI loans	279,173,000	294,292,000
Loans, Excluding PCI Loans [Member] | 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	77,834,000	89,106,000
Gross loans, excluding PCI loans	77,834,000	89,106,000
Loans, Excluding PCI Loans [Member] | Consumer and other loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
30-59 Days Past Due	64,000	75,000
60-89 Days Past Due	17,000	0
Total Past Due and Accruing	81,000	75,000
Nonaccrual	736,000	401,000
Current	69,067,000	54,627,000
Gross loans, excluding PCI loans	$ 69,884,000	$ 55,103,000