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BORROWINGS (Trust Preferred Securities) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Junior Subordinated Debentures [Member] |  |
Debt Instrument [Line Items] |  |
Face Amount | $ 71.0 | $ 71.0
Junior Subordinated Debentures [Member] | Capital Trust V Preferred Securities Due in 2037 [Member] |  |
Debt Instrument [Line Items] |  |
Face Amount | $ 50.0 |
Debt Instrument, interest rate, stated rate inclusive of interest rate swap - expired | 2.84% | 6.52%
Fixed rate | 3.07% |
Debt Instrument, interest rate, stated rate inclusive of interest rate swap - forward starting | 2.84% |
Junior Subordinated Debentures [Member] | Capital Trust V Preferred Securities Due in 2037 [Member] | LIBOR [Member] |  |
Debt Instrument [Line Items] |  |
Basis spread on variable rate | 1.48% |
Junior Subordinated Debentures [Member] | Slades Ferry Trust I Preferred Securities Due in 2034 [Member] |  |
Debt Instrument [Line Items] |  |
Face Amount | $ 10.0 |
Fixed rate | 4.39% |
Junior Subordinated Debentures [Member] | Slades Ferry Trust I Preferred Securities Due in 2034 [Member] | LIBOR [Member] |  |
Debt Instrument [Line Items] |  |
Basis spread on variable rate | 2.79% |
Junior Subordinated Debentures [Member] | Central Bancorp Capital Trust I Securities Due in 2034 [Member] |  |
Debt Instrument [Line Items] |  |
Face Amount | $ 5.1 |
Fixed rate | 4.03% |
Junior Subordinated Debentures [Member] | Central Bancorp Capital Trust I Securities Due in 2034 [Member] | LIBOR [Member] |  |
Debt Instrument [Line Items] |  |
Basis spread on variable rate | 2.44% |
Junior Subordinated Debentures [Member] | Central Bancorp Capital Trust II Securities Due in 2037 [Member] |  |
Debt Instrument [Line Items] |  |
Face Amount | $ 5.9 |
Fixed rate | 7.015% |
Junior Subordinated Debentures [Member] | Central Bancorp Capital Trust II Securities Due in 2037 [Member] | LIBOR [Member] |  |
Debt Instrument [Line Items] |  |
Basis spread on variable rate | 1.65% |
Fixed rate | 3.24% |
Subordinated Debt [Member] | Subordinated Debentures Due in November 2019 [Member] [Domain] |  |
Debt Instrument [Line Items] |  |
Fixed rate | 4.75% |
Subordinated Debt [Member] | Subordinated Debentures Due in November 2019 [Member] [Domain] | LIBOR [Member] |  |
Debt Instrument [Line Items] |  |
Basis spread on variable rate | 2.98% |

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