BORROWINGS (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Repurchase Agreements	$ 265,627	$ 230,612
Total short-term borrowings	538,997	530,129
Long-term Federal Home Loan Bank Advances	10,000	10,000
Total long-term borrowings	10,738	10,791
Short-term Debt [Member]
Debt Instrument [Line Items]
Repurchase Agreements	265,627	225,605
FHLBB advances less than 90 days	270,000	210,000
FHLBB and correspondent bank overnight borrowings	3,300	89,450
Capital lease obligation	70	67
Long-term Debt [Member]
Debt Instrument [Line Items]
Capital lease obligation	738	791
Commercial and Industrial Sector [Member] | Short-term Debt [Member]
Debt Instrument [Line Items]
Repurchase Agreements	$ 0	$ 5,007