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, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Description of assets
Total assets	$ 2,159	$ 3,159
Impaired loans-non-covered [Member] \| Commercial and industrial [Member]
Description of assets
Total assets	911	627
Impaired loans-non-covered [Member] \| Commercial real estate [Member]
Description of assets
Total assets	117
Impaired loans-non-covered [Member] \| Dairy & livestock and agribusiness [Member]
Description of assets
Total assets	1,061	2,096
Impaired loans-non-covered [Member] \| Consumer and other loans [Member]
Description of assets
Total assets	5	2
Covered Loans [Member]
Description of assets
Total assets	65	434
Level 1 [Member]
Description of assets
Total assets
Level 1 [Member] \| Impaired loans-non-covered [Member] \| Commercial and industrial [Member]
Description of assets
Total assets
Level 1 [Member] \| Impaired loans-non-covered [Member] \| Commercial real estate [Member]
Description of assets
Total assets
Level 1 [Member] \| Impaired loans-non-covered [Member] \| Dairy & livestock and agribusiness [Member]
Description of assets
Total assets
Level 1 [Member] \| Impaired loans-non-covered [Member] \| Consumer and other loans [Member]
Description of assets
Total assets
Level 1 [Member] \| Covered Loans [Member]
Description of assets
Total assets
Level 2 [Member]
Description of assets
Total assets
Level 2 [Member] \| Impaired loans-non-covered [Member] \| Commercial and industrial [Member]
Description of assets
Total assets
Level 2 [Member] \| Impaired loans-non-covered [Member] \| Commercial real estate [Member]
Description of assets
Total assets
Level 2 [Member] \| Impaired loans-non-covered [Member] \| Dairy & livestock and agribusiness [Member]
Description of assets
Total assets
Level 2 [Member] \| Impaired loans-non-covered [Member] \| Consumer and other loans [Member]
Description of assets
Total assets
Level 2 [Member] \| Covered Loans [Member]
Description of assets
Total assets
Level 3 [Member]
Description of assets
Total assets	6,270	12,934
Level 3 [Member] \| Impaired loans-non-covered [Member] \| Commercial and industrial [Member]
Description of assets
Total assets	2,998	529
Level 3 [Member] \| Impaired loans-non-covered [Member] \| Commercial real estate [Member]
Description of assets
Total assets	1,518
Level 3 [Member] \| Impaired loans-non-covered [Member] \| Dairy & livestock and agribusiness [Member]
Description of assets
Total assets	1,463	11,899
Level 3 [Member] \| Impaired loans-non-covered [Member] \| Consumer and other loans [Member]
Description of assets
Total assets	16	2
Level 3 [Member] \| Covered Loans [Member]
Description of assets
Total assets	275	504
Carrying Value [Member]
Description of assets
Total assets	6,270	12,934
Carrying Value [Member] \| Impaired loans-non-covered [Member] \| Commercial and industrial [Member]
Description of assets
Total assets	2,998	529
Carrying Value [Member] \| Impaired loans-non-covered [Member] \| Commercial real estate [Member]
Description of assets
Total assets	1,518
Carrying Value [Member] \| Impaired loans-non-covered [Member] \| Dairy & livestock and agribusiness [Member]
Description of assets
Total assets	1,463	11,899
Carrying Value [Member] \| Impaired loans-non-covered [Member] \| Consumer and other loans [Member]
Description of assets
Total assets	16	2
Carrying Value [Member] \| Covered Loans [Member]
Description of assets
Total assets	$ 275	$ 504