Shareholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Aug. 21, 2007
Class of Stock [Line Items]
Repurchase of common stock					500,000
Stock repurchase plan percentage of common stock					3.20%
Common stock, shares outstanding	22,955,963	22,867,802			15,814,662
Repurchased shares outstanding	166,600
Market value of common stock	$ 2,225,000	$ 5,988,000	$ 2,868,000
Maximum [Member]
Class of Stock [Line Items]
Cash dividend payable by bank	85,254,000
Tri Counties Bank [Member]
Class of Stock [Line Items]
Cash dividends received	$ 19,236,000	$ 16,758,000	$ 13,304,000
Stock Repurchase Plan [Member]
Class of Stock [Line Items]
Stock repurchase plan, number of shares repurchased	0
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	22,955,963	22,867,802	22,775,173	22,714,964
Stock repurchase plan, number of shares repurchased	107,390	264,800	106,355
Company's common stock in lieu of cash to exercise options to purchase shares	107,390	264,800	106,355
Market value of common stock	$ 3,854,000	$ 7,879,000	$ 2,868,000