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EMPLOYEE BENEFIT PLANS EMPLOYEE BENEFIT PLANS - Narrative (Details) |  | 12 Months Ended |
--- | --- | --- | ---
Aug. 01, 2014 USD ($) | Dec. 31, 2016 USD ($) acquisition employee | Dec. 31, 2015 USD ($) acquisition | Dec. 31, 2014 USD ($) | Dec. 31, 2011 acquisition | Apr. 30, 2015 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Non-elective contribution by employer as percentage of gross annual wages (percent) |  | 3.00% |  |  |  |
Matching contribution by employer (percent) |  | 100.00% |  |  |  |
Percentage of gross annual wages matched by employer (percent) |  | 4.00% |  |  |  |
Expense under the plan |  | $ 3,900,000 | $ 3,600,000 | $ 3,000,000 |  |
Number of significant acquisitions completed (acquisition) | acquisition |  | 3 | 2 |  | 2 |
Hampden Bancorp Inc |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Split-dollar agreement, accrued liability |  | $ 1,200,000 |  |  |  | $ 860,000
Fixed Income Funds |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Target plan asset allocation (percent) |  | 36.00% |  |  |  |
Marketable equity securities |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Target plan asset allocation (percent) |  | 63.00% |  |  |  |
Cash and Cash Equivalents |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Target plan asset allocation (percent) |  | 1.00% |  |  |  |
Pension Plans |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Number of employees participation (employee) | employee |  | 0 |  |  |  |
Net actuarial (gain) loss |  | $ 1,500,000 | $ 2,000,000 |  |  |
Expected cash contributions by employer |  | 0 |  |  |  |
Amount expected to be amortized from other comprehensive income into net periodic pension cost over the next fiscal year |  | 100,000 |  |  |  |
Defined benefit plan, fair value of plan assets |  | 5,121,000 | 5,211,000 | 5,756,000 |  |
Transfers between Level 1, 2, and 3 |  | $ 0 | $ 0 |  |  |
Discount rate (percent) |  | 4.17% | 3.82% |  |  |
Pension Plans | Level 3 |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined benefit plan, fair value of plan assets |  | $ 0 | $ 0 |  |  |
Pension Plans | Fixed Income Funds |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Defined benefit plan, fair value of plan assets |  | 1,378,000 | 1,564,000 |  |  |
Postretirement Benefits |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Net actuarial (gain) loss |  | (126,000) | (257,000) |  |  |
Amount expected to be amortized from other comprehensive income into net periodic pension cost over the next fiscal year |  | 83,000 |  |  |  |
Defined benefit plan, fair value of plan assets |  | 0 | 0 | 0 |  |
Prior service cost of long-term care plan participants | $ 558,000 | $ 0 | $ 1,595,000 |  |  |
Discount rate (percent) |  | 3.91% | 4.15% |  |  |
Assumed health care cost trend rate (percent) |  | 8.15% |  |  |  |
Assumed health care cost trend rate in 2020 (percent) |  | 3.89% |  |  |  |
Supplemental Employee Retirement Plan |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Eligible age for retirement plan |  | 62 years |  |  |  |
Accrued liability for other plans |  | $ 7,400,000 | $ 6,500,000 |  |  |
Other plans expense |  | 917,000 | $ 752,000 | $ 583,000 |  |
Supplemental Employee Retirement Plan | Hampden Bancorp Inc |  |  |  |  |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |  |  |  |  |
Accrued liability for other plans |  | $ 1,200,000 |  |  |  | $ 1,400,000

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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6749337 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6520858 + Details
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- Definition Amount of employer's best estimate of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 + Details
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