Shareholders' Equity - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	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	22,941,464		22,941,464		22,867,802		15,814,662
Repurchased shares outstanding	166,600		166,600
Market value of common stock			$ 3,854,000	$ 2,774,000
Maximum [Member]
Class of Stock [Line Items]
Cash dividend payable by bank					$ 82,615,000
Tri Counties Bank [Member]
Class of Stock [Line Items]
Cash dividends received	$ 5,185,000	$ 4,097,000	$ 14,394,000	$ 11,434,000
Stock Repurchase Plan [Member]
Class of Stock [Line Items]
Stock repurchase plan, number of shares repurchased			0		0
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	22,941,464	22,827,277	22,941,464	22,827,277	22,867,802	22,775,173
Stock repurchase plan, number of shares repurchased			107,390	101,125
Company's common stock in lieu of cash to exercise options to purchase shares	21,738	4,129	107,390	101,125
Market value of common stock	$ 762,000	$ 108,000	$ 3,854,000	$ 2,774,000