Shareholders' Equity - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 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	30,417,818		30,417,818		22,955,963		15,814,662
Market value of shares repurchased			$ 1,124,000	$ 3,854,000
Maximum [Member]
Class of Stock [Line Items]
Amount available to dividend to holding company					$ 85,254,000
Tri Counties Bank [Member]
Class of Stock [Line Items]
Cash dividends received	$ 13,507,000	$ 5,185,000	$ 22,649,000	$ 14,394,000
2007 Stock Repurchase Plan [Member]
Class of Stock [Line Items]
Repurchased shares outstanding	166,600		166,600
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	30,417,818	22,941,464	30,417,818	22,941,464	22,955,963	22,867,802
Company's common stock in lieu of cash to exercise options to purchase shares	11,630	21,738	28,850	107,390
Market value of shares repurchased	$ 453,000	$ 762,000	$ 1,124,000	$ 3,854,000