Shareholders' Equity - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 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	22,956,323		22,955,963		15,814,662
Repurchased shares outstanding	166,600
Market value of common stock	$ 4,000	$ 604,000
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	$ 4,372,000	$ 4,042,000
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	22,956,323	22,873,305	22,955,963	22,867,802
Company's common stock in lieu of cash to exercise options to purchase shares	134	16,251
Market value of common stock	$ 4,000	$ 604,000