Shareholders' Equity - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	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	23,004,153		23,004,153		22,955,963		15,814,662
Repurchased shares outstanding	166,600		166,600
Market value of common stock			$ 671,000	$ 3,092,000
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	$ 4,770,000	$ 5,167,000	$ 9,142,000	$ 9,209,000
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	23,004,153	22,925,069	23,004,153	22,925,069	22,955,963	22,867,802
Company's common stock in lieu of cash to exercise options to purchase shares	17,086	69,401	17,220	85,652
Market value of common stock	$ 667,000	$ 2,488,000	$ 671,000	$ 3,092,000