Loans and Lease Finance Receivables and Allowance for Loan Losses - Schedule of Recorded Investment in, and Aging of, Past Due and Nonaccrual Loans, Including PCI Loans by Class of Loans (Detail) - USD ($)

Loans and Lease Finance Receivables and Allowance for Loan Losses - Schedule of Recorded Investment in, and Aging of, Past Due and Nonaccrual Loans, Including PCI Loans by Class of Loans (Detail) - USD ($)	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current		$ 7,561,389,000
Total Gross loans	$ 7,568,319,000		$ 7,769,439,000
30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	59,000		33,000
60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Nonaccrual	1,100,000		57,000
Loans, Excluding PCI Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		1,653,000	5,290,000
Nonaccrual		5,277,000	19,951,000
Current			7,726,984,000
Total Gross loans	7,568,319,000	7,568,319,000	7,752,225,000
Loans, Excluding PCI Loans [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		878,000	4,431,000
Loans, Excluding PCI Loans [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		775,000	859,000
Loans, Excluding PCI Loans [Member] \| Commercial and Industrial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		2,000	909,000
Nonaccrual		1,266,000	7,490,000
Current		933,859,000	993,810,000
Total Gross loans		935,127,000	1,002,209,000
Loans, Excluding PCI Loans [Member] \| Commercial and Industrial [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		2,000	820,000
Loans, Excluding PCI Loans [Member] \| Commercial and Industrial [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing	$ 0		89,000
Loans, Excluding PCI Loans [Member] \| Small Business Administration [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		1,402,000	1,307,000
Nonaccrual		2,032,000	2,892,000
Current		301,574,000	345,844,000
Total Gross loans		305,008,000	350,043,000
Loans, Excluding PCI Loans [Member] \| Small Business Administration [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		870,000	1,172,000
Loans, Excluding PCI Loans [Member] \| Small Business Administration [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		532,000	135,000
Loans, Excluding PCI Loans [Member] \| Commercial Real Estate Owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	2,789,000
Nonaccrual		479,000	589,000
Current		2,083,171,000	2,114,612,000
Total Gross loans		2,083,650,000	2,117,990,000
Loans, Excluding PCI Loans [Member] \| Commercial Real Estate Owner Occupied [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	2,439,000
Loans, Excluding PCI Loans [Member] \| Commercial Real Estate Owner Occupied [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	350,000
Loans, Excluding PCI Loans [Member] \| Commercial Real Estate Non-owner Occupied [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Nonaccrual		245,000	5,479,000
Current		3,290,722,000	3,270,760,000
Total Gross loans		3,290,967,000	3,276,239,000
Loans, Excluding PCI Loans [Member] \| Commercial Real Estate Non-owner Occupied [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| Commercial Real Estate Non-owner Occupied [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| Construction Speculative [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Nonaccrual		0	0
Current		106,895,000	118,233,000
Total Gross loans		106,895,000	118,233,000
Loans, Excluding PCI Loans [Member] \| Construction Speculative [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| Construction Speculative [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| Construction Non-speculative [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Nonaccrual		0	0
Current		10,030,000	4,549,000
Total Gross loans		10,030,000	4,549,000
Loans, Excluding PCI Loans [Member] \| Construction Non-speculative [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| Construction Non-speculative [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| SFR Mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		249,000	285,000
Nonaccrual		878,000	2,937,000
Current		282,341,000	293,282,000
Total Gross loans		283,468,000	296,504,000
Loans, Excluding PCI Loans [Member] \| SFR Mortgage [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		6,000	0
Loans, Excluding PCI Loans [Member] \| SFR Mortgage [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		243,000	285,000
Loans, Excluding PCI Loans [Member] \| Dairy & Livestock and Agribusiness [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Nonaccrual		0	78,000
Current		383,709,000	393,765,000
Total Gross loans		383,709,000	393,843,000
Loans, Excluding PCI Loans [Member] \| Dairy & Livestock and Agribusiness [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| Dairy & Livestock and Agribusiness [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| Municipal Lease Finance Receivables [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Nonaccrual		0	0
Current		53,146,000	64,186,000
Total Gross loans		53,146,000	64,186,000
Loans, Excluding PCI Loans [Member] \| Municipal Lease Finance Receivables [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| Municipal Lease Finance Receivables [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| Consumer and Other Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Nonaccrual		377,000	486,000
Current		115,942,000	127,943,000
Total Gross loans		116,319,000	128,429,000
Loans, Excluding PCI Loans [Member] \| Consumer and Other Loans [Member] \| 30-59 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		0	0
Loans, Excluding PCI Loans [Member] \| Consumer and Other Loans [Member] \| 60-89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total Past Due and Accruing		$ 0	$ 0