Fair Value Information - Assets and Liabilities Measured at Fair Value on Non-Recurring Basis (Detail) - Fair Value on Non-Recurring Basis [Member] - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Description of assets
Total assets	$ 14	$ 74
Other Real Estate Owned [Member]
Description of assets
Total assets	0	0
Loans, Excluding PCI Loans [Member] | Asset Held-for-Sale [Member]
Description of assets
Total assets	0	0
Loans, Excluding PCI Loans [Member] | Commercial and Industrial [Member]
Description of assets
Total assets	0	0
Loans, Excluding PCI Loans [Member] | Small Business Administration [Member]
Description of assets
Total assets	0	0
Loans, Excluding PCI Loans [Member] | Commercial Real Estate [Member]
Description of assets
Total assets	0	0
Loans, Excluding PCI Loans [Member] | Construction [Member]
Description of assets
Total assets	0	0
Loans, Excluding PCI Loans [Member] | SFR Mortgage [Member]
Description of assets
Total assets	13	0
Loans, Excluding PCI Loans [Member] | Dairy & Livestock and Agribusiness [Member]
Description of assets
Total assets	0	0
Loans, Excluding PCI Loans [Member] | Consumer and Other Loans [Member]
Description of assets
Total assets	1	74
Level 1 [Member]
Description of assets
Total assets	0	0
Level 1 [Member] | Other Real Estate Owned [Member]
Description of assets
Total assets	0	0
Level 1 [Member] | Loans, Excluding PCI Loans [Member] | Asset Held-for-Sale [Member]
Description of assets
Total assets	0	0
Level 1 [Member] | Loans, Excluding PCI Loans [Member] | Commercial and Industrial [Member]
Description of assets
Total assets	0	0
Level 1 [Member] | Loans, Excluding PCI Loans [Member] | Small Business Administration [Member]
Description of assets
Total assets	0	0
Level 1 [Member] | Loans, Excluding PCI Loans [Member] | Commercial Real Estate [Member]
Description of assets
Total assets	0	0
Level 1 [Member] | Loans, Excluding PCI Loans [Member] | Construction [Member]
Description of assets
Total assets	0	0
Level 1 [Member] | Loans, Excluding PCI Loans [Member] | SFR Mortgage [Member]
Description of assets
Total assets	0	0
Level 1 [Member] | Loans, Excluding PCI Loans [Member] | Dairy & Livestock and Agribusiness [Member]
Description of assets
Total assets	0	0
Level 1 [Member] | Loans, Excluding PCI Loans [Member] | Consumer and Other Loans [Member]
Description of assets
Total assets	0	0
Level 2 [Member]
Description of assets
Total assets	0	0
Level 2 [Member] | Other Real Estate Owned [Member]
Description of assets
Total assets	0	0
Level 2 [Member] | Loans, Excluding PCI Loans [Member] | Asset Held-for-Sale [Member]
Description of assets
Total assets	0	0
Level 2 [Member] | Loans, Excluding PCI Loans [Member] | Commercial and Industrial [Member]
Description of assets
Total assets	0	0
Level 2 [Member] | Loans, Excluding PCI Loans [Member] | Small Business Administration [Member]
Description of assets
Total assets	0	0
Level 2 [Member] | Loans, Excluding PCI Loans [Member] | Commercial Real Estate [Member]
Description of assets
Total assets	0	0
Level 2 [Member] | Loans, Excluding PCI Loans [Member] | Construction [Member]
Description of assets
Total assets	0	0
Level 2 [Member] | Loans, Excluding PCI Loans [Member] | SFR Mortgage [Member]
Description of assets
Total assets	0	0
Level 2 [Member] | Loans, Excluding PCI Loans [Member] | Dairy & Livestock and Agribusiness [Member]
Description of assets
Total assets	0	0
Level 2 [Member] | Loans, Excluding PCI Loans [Member] | Consumer and Other Loans [Member]
Description of assets
Total assets	0	0
Level 3 [Member]
Description of assets
Total assets	14	378
Level 3 [Member] | Other Real Estate Owned [Member]
Description of assets
Total assets	0	0
Level 3 [Member] | Loans, Excluding PCI Loans [Member] | Asset Held-for-Sale [Member]
Description of assets
Total assets	0	0
Level 3 [Member] | Loans, Excluding PCI Loans [Member] | Commercial and Industrial [Member]
Description of assets
Total assets	0	0
Level 3 [Member] | Loans, Excluding PCI Loans [Member] | Small Business Administration [Member]
Description of assets
Total assets	0	0
Level 3 [Member] | Loans, Excluding PCI Loans [Member] | Commercial Real Estate [Member]
Description of assets
Total assets	0	0
Level 3 [Member] | Loans, Excluding PCI Loans [Member] | Construction [Member]
Description of assets
Total assets	0	0
Level 3 [Member] | Loans, Excluding PCI Loans [Member] | SFR Mortgage [Member]
Description of assets
Total assets	13	0
Level 3 [Member] | Loans, Excluding PCI Loans [Member] | Dairy & Livestock and Agribusiness [Member]
Description of assets
Total assets	0	0
Level 3 [Member] | Loans, Excluding PCI Loans [Member] | Consumer and Other Loans [Member]
Description of assets
Total assets	1	378
Carrying Value [Member]
Description of assets
Total assets	14	378
Carrying Value [Member] | Other Real Estate Owned [Member]
Description of assets
Total assets	0	0
Carrying Value [Member] | Loans, Excluding PCI Loans [Member] | Asset Held-for-Sale [Member]
Description of assets
Total assets	0	0
Carrying Value [Member] | Loans, Excluding PCI Loans [Member] | Commercial and Industrial [Member]
Description of assets
Total assets	0	0
Carrying Value [Member] | Loans, Excluding PCI Loans [Member] | Small Business Administration [Member]
Description of assets
Total assets	0	0
Carrying Value [Member] | Loans, Excluding PCI Loans [Member] | Commercial Real Estate [Member]
Description of assets
Total assets	0	0
Carrying Value [Member] | Loans, Excluding PCI Loans [Member] | Construction [Member]
Description of assets
Total assets	0	0
Carrying Value [Member] | Loans, Excluding PCI Loans [Member] | SFR Mortgage [Member]
Description of assets
Total assets	13	0
Carrying Value [Member] | Loans, Excluding PCI Loans [Member] | Dairy & Livestock and Agribusiness [Member]
Description of assets
Total assets	0	0
Carrying Value [Member] | Loans, Excluding PCI Loans [Member] | Consumer and Other Loans [Member]
Description of assets
Total assets	$ 1	$ 378