BORROWINGS (Details Textual) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Subordinated Debt	$ 34,682,000				$ 34,635,000				$ 34,682,000	$ 34,635,000
Interest Expense, Long-term Debt									4,000,000	5,800,000	$ 6,600,000
Gains (Losses) on Extinguishment of Debt	0	$ 0	$ 0	$ 0	0	$ 0	$ 0	$ (437,000)	0	(437,000)	(122,000)
Loans Pledged as Collateral	$ 1,500,000,000				$ 1,400,000,000				$ 1,500,000,000	$ 1,400,000,000
Federal Home Loan Bank Advances Weighted Average Interest Rate inclusive of Swaps	2.47%				3.42%				2.47%	3.42%
Repayments of Debt										$ 49,000,000	13,000,000
Securities Sold under Agreements to Repurchase	$ 162,679,000				$ 176,913,000				$ 162,679,000	176,913,000
Interest expense on short-term borrowings									257,000	208,000	$ 210,000
Total Long-Term Borrowings	107,755,000				107,742,000				107,755,000	107,742,000
Parent Company [Member]
Debt Instrument [Line Items]
Subordinated Debt	35,000,000				35,000,000				35,000,000	35,000,000
Capital Trust V Preferred Securities Due in 2037 [Member] | Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Total Long-Term Borrowings	$ 51,503,000				$ 51,500,000				$ 51,503,000	$ 51,500,000
Fixed rate	3.07%								3.07%
Debt Instrument, interest rate, stated rate inclusive of interest rate swap - expired	2.84%				6.52%				2.84%	6.52%
Debt Instrument, interest rate, stated rate inclusive of interest rate swap - forward starting	2.84%								2.84%
Capital Trust V Preferred Securities Due in 2037 [Member] | Junior Subordinated Debt [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate									1.48%
Subordinated Debentures Due in November 2019 [Member] [Domain] | Subordinated Debentures [Member]
Debt Instrument [Line Items]
Fixed rate	4.75%								4.75%
Subordinated Debentures Due in November 2019 [Member] [Domain] | Subordinated Debentures [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate									2.98%
Slades Ferry Trust I Preferred Securities Due in 2034 [Member] | Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Total Long-Term Borrowings	$ 10,229,000				$ 10,224,000				$ 10,229,000	$ 10,224,000
Fixed rate	4.39%								4.39%
Slades Ferry Trust I Preferred Securities Due in 2034 [Member] | Junior Subordinated Debt [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate									2.79%
Central Bancorp Capital Trust I Securities Due in 2034 [Member] | Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Total Long-Term Borrowings	$ 5,258,000				5,302,000				$ 5,258,000	5,302,000
Fixed rate	4.03%								4.03%
Central Bancorp Capital Trust I Securities Due in 2034 [Member] | Junior Subordinated Debt [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Basis spread on variable rate									2.44%
Central Bancorp Capital Trust II Securities Due in 2037 [Member] | Junior Subordinated Debt [Member]
Debt Instrument [Line Items]
Total Long-Term Borrowings	$ 6,083,000				6,081,000				$ 6,083,000	6,081,000
Fixed rate	7.015%								7.015%
Central Bancorp Capital Trust II Securities Due in 2037 [Member] | Junior Subordinated Debt [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Fixed rate	3.24%								3.24%
Basis spread on variable rate									1.65%
Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Federal home loan bank unused remaining available borrowing capacity	$ 954,800,000				$ 793,100,000				$ 954,800,000	$ 793,100,000
Line of credit maximum borrowing capacity	$ 5,000,000								$ 5,000,000