Loans and Lease Finance Receivables and Allowance for Loan Losses - Summary of Loans Modified as Troubled Debt Restructurings (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014 Contract	Dec. 31, 2013 Contract	Dec. 31, 2012 Contract
American Security Bank [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	6
Interest rate reduction [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	3
Loans [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	8	18	31
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	$ 4,746	$ 29,069	$ 37,221
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	974	29,069	37,221
Troubled debt restructurings, Outstanding Recorded Investment	4,597	24,719	34,956
Troubled debt restructurings, Financial Effect Resulting From Modifications	185	244	3
Commercial and industrial [Member] | Interest rate reduction [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	3	0	1
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	553	0	80
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	553	0	80
Troubled debt restructurings, Outstanding Recorded Investment	522	0	66
Troubled debt restructurings, Financial Effect Resulting From Modifications	185	0	0
Commercial and industrial [Member] | Change in amortization period or maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	0	4	8
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	0	621	2,301
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	0	621	2,301
Troubled debt restructurings, Outstanding Recorded Investment	0	570	1,817
Troubled debt restructurings, Financial Effect Resulting From Modifications	0	95	3
Commercial real estate Owner occupied [Member] | Interest rate reduction [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	1	0	0
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	199	0	0
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	0	0	0
Troubled debt restructurings, Outstanding Recorded Investment	187	0	0
Troubled debt restructurings, Financial Effect Resulting From Modifications	0	0	0
Commercial real estate Owner occupied [Member] | Change in amortization period or maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	0	1	6
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	0	168	4,225
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	0	168	4,225
Troubled debt restructurings, Outstanding Recorded Investment	0	138	3,903
Troubled debt restructurings, Financial Effect Resulting From Modifications	0	0	0
Commercial real estate Owner occupied [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	0
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	0
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	0
Troubled debt restructurings, Outstanding Recorded Investment	0
Troubled debt restructurings, Financial Effect Resulting From Modifications	0
Commercial real estate Non-owner occupied [Member] | Interest rate reduction [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	3	0	1
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	3,573	0	3,378
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	0	0	3,378
Troubled debt restructurings, Outstanding Recorded Investment	3,469	0	3,359
Troubled debt restructurings, Financial Effect Resulting From Modifications	0	0	0
Commercial real estate Non-owner occupied [Member] | Change in amortization period or maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	0	0	4
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	0	0	5,906
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	0	0	5,906
Troubled debt restructurings, Outstanding Recorded Investment	0	0	5,303
Troubled debt restructurings, Financial Effect Resulting From Modifications	0	0	0
Commercial real estate Non-owner occupied [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	0
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	0
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	0
Troubled debt restructurings, Outstanding Recorded Investment	0
Troubled debt restructurings, Financial Effect Resulting From Modifications	0
Construction Speculative [Member] | Interest rate reduction [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans		0	0
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment		0	0
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment		0	0
Troubled debt restructurings, Outstanding Recorded Investment		0	0
Troubled debt restructurings, Financial Effect Resulting From Modifications		0	0
Construction Speculative [Member] | Change in amortization period or maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans		0	1
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment		0	10,966
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment		0	10,966
Troubled debt restructurings, Outstanding Recorded Investment		0	10,663
Troubled debt restructurings, Financial Effect Resulting From Modifications		0	0
Construction Non-speculative [Member] | Interest rate reduction [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans			0
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment			0
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment			0
Troubled debt restructurings, Outstanding Recorded Investment			0
Troubled debt restructurings, Financial Effect Resulting From Modifications			0
Construction Non-speculative [Member] | Change in amortization period or maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans			0
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment			0
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment			0
Troubled debt restructurings, Outstanding Recorded Investment			0
Troubled debt restructurings, Financial Effect Resulting From Modifications			0
Construction Non-speculative [Member] | Other [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans			0
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment			0
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment			0
Troubled debt restructurings, Outstanding Recorded Investment			0
Troubled debt restructurings, Financial Effect Resulting From Modifications			0
Single Family Mortgage Loans [Member] | Interest rate reduction [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans		3	1
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment		1,365	399
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment		1,365	399
Troubled debt restructurings, Outstanding Recorded Investment		1,349	398
Troubled debt restructurings, Financial Effect Resulting From Modifications		0	0
Single Family Mortgage Loans [Member] | Change in amortization period or maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans		0	0
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment		0	0
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment		0	0
Troubled debt restructurings, Outstanding Recorded Investment		0	0
Troubled debt restructurings, Financial Effect Resulting From Modifications		0	0
Dairy & livestock and agribusiness [Member] | Interest rate reduction [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	0	0	0
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	0	0	0
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	0	0	0
Troubled debt restructurings, Outstanding Recorded Investment	0	0	0
Troubled debt restructurings, Financial Effect Resulting From Modifications	0	0	0
Dairy & livestock and agribusiness [Member] | Change in amortization period or maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	0	10	7
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	0	26,915	9,447
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	0	26,915	9,447
Troubled debt restructurings, Outstanding Recorded Investment	0	22,662	9,184
Troubled debt restructurings, Financial Effect Resulting From Modifications	0	149	0
Municipal lease finance receivables [Member] | Interest rate reduction [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans			0
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment			0
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment			0
Troubled debt restructurings, Outstanding Recorded Investment			0
Troubled debt restructurings, Financial Effect Resulting From Modifications			0
Municipal lease finance receivables [Member] | Change in amortization period or maturity [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans			2
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment			519
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment			519
Troubled debt restructurings, Outstanding Recorded Investment			263
Troubled debt restructurings, Financial Effect Resulting From Modifications			0
Consumer [Member] | Interest rate reduction [Member]
Financing Receivable, Modifications [Line Items]
Troubled debt restructurings, Number of Loans	1
Troubled debt restructurings, Pre-Modification Outstanding Recorded Investment	421
Troubled debt restructurings, Post-Modification Outstanding Recorded Investment	421
Troubled debt restructurings, Outstanding Recorded Investment	419
Troubled debt restructurings, Financial Effect Resulting From Modifications	$ 0