COMMITMENTS AND CONTINGENCIES AND OTHER MATTERS (Detail Textuals) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Commitments and Contingencies [Line Items]
Rental expenses under leases	$ 6,800	$ 5,300	$ 3,400
Federal funds purchased	100,000	$ 120,000
Bridgehampton National Bank
Commitments and Contingencies [Line Items]
Required cash balance with Federal Reserve Bank of New York	7,300
Average balance maintained	23,100
Federal funds purchased	100,000
Amount available for transactions under Master Repurchase Agreement	1,220,000
Bridgehampton National Bank | Overnight line of credit with the Federal Home Loan Bank of New York
Commitments and Contingencies [Line Items]
Lines of credit with unaffiliated correspondent banks to provide short-term credit	349,500
Lines of credit available on an unsecured basis	$ 329,500