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ACQUISITIONS (Details) shares in Thousands, $ in Thousands |  |  |  |  |  |  |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 04, 2017 USD ($) shares | Nov. 17, 2017 USD ($) | May 12, 2017 USD ($) Branch shares | Mar. 01, 2017 USD ($) | Feb. 03, 2017 USD ($) | Jan. 02, 2017 USD ($) | Dec. 04, 2015 USD ($) Branch shares | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  |  |  |  |  |  |  |  | $ 107,414 | $ 575 | $ (25,505)
Goodwill |  |  |  |  |  |  |  |  | $ 734,430 |  |  | 734,430 | $ 465,142 | $ 463,252
Increase (decrease) in other assets as a result of reclassification and adjustment |  |  |  |  |  |  |  |  | (300) | $ 5,500 | $ (2,700) |  |  |
Increase (decrease) in other liabilities as a result of reclassification and adjustment |  |  |  |  |  |  |  |  | (20) | 6,600 | (2,400) |  |  |
Increase in investments as a result of updated information not available at the time of acquisition |  |  |  |  |  |  |  |  |  | 200 |  |  |  |
Decrease in loans as a result of an adjustment to the valuation of acquired impaired loans |  |  |  |  |  |  |  |  | (500) | (600) |  |  |  |
Increase in premises and equipment as a result of updated appraisal information not available at the time of acquisition |  |  |  |  |  |  |  |  |  | 3,600 |  |  |  |
Gordon B. Roberts Agency, Inc [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  | $ 1,350 |  |  |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  | $ 3,700 |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares (in shares) | shares |  | 40 |  |  |  |  |  |  |  |  |  |  |  |
Assets acquired |  | $ 600 |  |  |  |  |  |  |  |  |  |  |  |
Other liabilities |  | 700 |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  | 2,200 |  |  |  |  |  |  |  |  |  |  |  |
Gordon B. Roberts Agency, Inc [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  | $ 1,600 |  |  |  |  |  |  |  |  |  |  |  |
Northeast Capital Management, Inc [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  | $ 1,200 |  |  |  |  |  |  |  |  |  |  |
Contingent payment |  |  | 200 |  |  |  |  |  |  |  |  |  |  |
Northeast Capital Management, Inc [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  | $ 1,200 |  |  |  |  |  |  |  |  |  |  |
Dryfoos [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  | $ 3,000 |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  | 1,700 |  |  |  |  |  |  |  |  |
Dryfoos [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  | $ 1,700 |  |  |  |  |  |  |  |  |
BAS [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  |  |  | $ 1,200 |  |  |  |  |  |  |
BAS [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  |  | $ 1,200 |  |  |  |  |  |  |
Merchants Bancshares, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash | [1] |  |  | $ 82,898 |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  |  | $ 345,152 |  |  |  |  |  |  |  |  |  |
Issuance of common shares (in shares) | shares |  |  |  | 4,680 |  |  |  |  |  |  |  |  |  |
Assets acquired |  |  |  | $ 2,000,000 |  |  |  |  |  |  |  |  |  |
Other liabilities |  |  |  | 11,750 |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  | 189,046 |  |  |  |  |  |  |  |  |  |
Revenue earned |  |  |  |  |  |  |  |  |  |  |  | 42,600 |  |
Expenses incurred |  |  |  |  |  |  |  |  |  |  |  | $ 19,900 |  |
Increase (decrease) in goodwill as a result of fair value adjustment |  |  |  |  |  |  |  |  | $ 800 | (2,000) |  |  |  |
Merchants Bancshares, Inc. [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  | $ 2,857 |  |  |  |  |  |  |  |  |  |
Merchants Bancshares, Inc. [Member] | Vermont [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of branch locations added upon merger | Branch |  |  |  | 31 |  |  |  |  |  |  |  |  |  |
Merchants Bancshares, Inc. [Member] | Massachusetts [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of branch locations added upon merger | Branch |  |  |  | 1 |  |  |  |  |  |  |  |  |  |
Oneida Financial Corp [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash | [1] |  |  |  |  |  |  | $ 56,266 |  |  |  |  |  |
Purchase price of acquisition |  |  |  |  |  |  |  | $ 158,468 |  |  |  |  |  |
Issuance of common shares (in shares) | shares |  |  |  |  |  |  |  | 2,380 |  |  |  |  |  |
Number of new branch locations included in acquisition or purchase agreement | Branch |  |  |  |  |  |  |  | 12 |  |  |  |  |  |
Assets acquired |  |  |  |  |  |  |  | $ 769,400 |  |  |  |  |  |
Other liabilities |  |  |  |  |  |  |  | 1,333 |  |  |  |  |  |
Goodwill |  |  |  |  |  |  |  | 89,681 |  |  |  |  |  |
Oneida Financial Corp [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  |  |  | $ 9,994 |  |  |  |  |  |
Northeast Retirement Services, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash | [1] |  |  |  |  | $ 70,073 |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  |  |  |  | 148,556 |  |  |  |  |  |  |  |
Tangible assets, net |  |  |  |  |  | $ 36,100 |  |  |  |  |  |  |  |
Factor of straight-line rate of depreciation used in declining balance depreciation method |  |  |  |  |  | 150.00% |  |  |  |  |  |  |  |
Period of amortization |  |  |  |  |  |  |  |  |  |  |  | 10 years |  |
Deferred tax liability |  |  |  |  |  | $ 24,200 |  |  |  |  |  |  |  |
Other liabilities |  |  |  |  |  | 28,002 |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  | 76,430 |  |  |  |  |  |  |  |
Revenue earned |  |  |  |  |  |  |  |  |  |  |  | $ 31,500 |  |
Expenses incurred |  |  |  |  |  |  |  |  |  |  |  | $ 21,500 |  |
Increase (decrease) in goodwill as a result of fair value adjustment |  |  |  |  |  |  |  |  |  | $ (100) | $ 300 |  |  |
Northeast Retirement Services, Inc. [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  | $ 60,200 |  |  |  |  |  |  |  |
[1] | Includes NECM $0.2 million contingent cash payment consideration.

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