Shareholders' Equity - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	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	22,867,802	22,775,173			15,814,662
Repurchased shares outstanding	166,600
Market value of common stock	$ 5,988,000	$ 2,868,000	$ 2,551,000
Maximum [Member]
Class of Stock [Line Items]
Cash dividend payable by bank	$ 82,615,000
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	22,867,802	22,775,173	22,714,964	16,076,662
Company's common stock in lieu of cash to exercise options to purchase shares	264,800	106,355	103,268
Market value of common stock	$ 7,879,000	$ 2,868,000	$ 2,551,000
Tri Counties Bank [Member]
Class of Stock [Line Items]
Cash dividends received	$ 16,758,000	$ 13,304,000	$ 8,270,000