Shareholders' Equity - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014	Aug. 21, 2007	Dec. 31, 2013
Class of Stock [Line Items]
Dividend payable by bank			$ 52,798,000
Shareholder Rights Plan	Jun. 25, 2001
Percentage of restriction on acquiring outstanding common stock by person or group	15.00%
Repurchase of common stock				500,000
Stock repurchase plan percentage of common stock				3.20%
Common stock, shares outstanding	22,740,503		22,714,964	15,814,662
Repurchased shares outstanding	166,600
Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares outstanding	22,740,503	16,120,297	22,714,964		16,076,662
Company's common stock in lieu of cash to exercise options to purchase shares	21,461	100,385
Market value of common stock	509,000	2,482,000
Mellon Investor Services LLC [Member] | Amendment [Member]
Class of Stock [Line Items]
Amendment date of Right Plan	Jun. 04, 2014
Schedule expiration date plan	Jul. 01, 2014
Tri Counties Bank [Member]
Class of Stock [Line Items]
Cash dividends received	$ 2,120,000	$ 2,050,000