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ACQUISITIONS (Details) shares in Thousands, $ in Thousands |  |  |  |  |  |  |  |  | 3 Months Ended | 6 Months Ended |  |
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Apr. 02, 2018 USD ($) | Jan. 02, 2018 USD ($) | Dec. 04, 2017 USD ($) shares | Nov. 17, 2017 USD ($) | May 12, 2017 USD ($) Branch Location shares | Mar. 01, 2017 USD ($) | Feb. 03, 2017 USD ($) | Jan. 01, 2017 USD ($) | Jun. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) | Jun. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) | Jan. 02, 2017 USD ($)
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  |  |  |  |  |  |  |  | $ 1,737 | $ 105,402 |  |
Goodwill |  |  |  |  |  |  |  |  | $ 733,479 |  | $ 733,604 | 733,479 | 733,604 | $ 734,430 |
HR Consultants (SA), LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash | $ 300 |  |  |  |  |  |  |  |  |  |  |  |  |  |
HR Consultants (SA), LLC [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles | $ 300 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Penna & Associates Agency, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  | $ 800 |  |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  | 300 |  |  |  |  |  |  |  |  |  |  |  |  |
Penna & Associates Agency, Inc. [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  | 300 |  |  |  |  |  |  |  |  |  |  |  |  |
Styles Bridges Associates [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  | 700 |  |  |  |  |  |  |  |  |  |  |  |  |
Styles Bridges Associates [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  | $ 700 |  |  |  |  |  |  |  |  |  |  |  |  |
Gordon B. Roberts Agency, Inc [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  | $ 3,700 |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  | $ 1,350 |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common shares (in shares) | shares |  |  | 40 |  |  |  |  |  |  |  |  |  |  |  |
Other liabilities |  |  | $ 600 |  |  |  |  |  |  |  |  |  |  |  |
Goodwill |  |  | 2,100 |  |  |  |  |  |  |  |  |  |  |  |
Assets acquired |  |  | 600 |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in other liabilities as a result of reclassification and adjustment |  |  |  |  |  |  |  |  | (90) |  |  |  |  |  |
Increase (decrease) in goodwill as a result of fair value adjustment |  |  |  |  |  |  |  |  | (90) |  |  |  |  |  |
Gordon B. Roberts Agency, Inc [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  | $ 1,600 |  |  |  |  |  |  |  |  |  |  |  |
Northeast Capital Management, Inc [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  | $ 1,200 |  |  |  |  |  |  |  |  |  |  |
Northeast Capital Management, Inc [Member] | Customer Lists [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  | $ 1,200 |  |  |  |  |  |  |  |  |  |  |
Merchants Bancshares, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  |  |  | $ 345,152 |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  | $ 82,898 |  |  |  |  |  |  |  |  |  |
Issuance of common shares (in shares) | shares |  |  |  |  | 4,680 |  |  |  |  |  |  |  |  |  |
Other liabilities |  |  |  |  | $ 11,750 |  |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  | 189,046 |  |  |  |  |  |  |  |  |  |
Assets acquired |  |  |  |  | $ 2,000,000 |  |  |  |  |  |  |  |  |  |
Revenues earned |  |  |  |  |  |  |  |  | 16,100 |  | 8,900 | 32,200 | 8,900 |  |
Expenses incurred |  |  |  |  |  |  |  |  | 7,600 |  | 4,000 | 15,400 | 4,000 |  |
Merchants Bancshares, Inc. [Member] | VERMONT [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of branch locations added upon merger | Branch |  |  |  |  | 31 |  |  |  |  |  |  |  |  |  |
Merchants Bancshares, Inc. [Member] | Massachusetts [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of branch locations added upon merger | Location |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |
Merchants Bancshares, Inc. [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  | $ 2,857 |  |  |  |  |  |  |  |  |  |
Dryfoos [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  |  | $ 3,000 |  |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  | 1,700 |  |  |  |  |  |  |  |  |
Dryfoos [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  | $ 1,700 |  |  |  |  |  |  |  |  |
BAS [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash |  |  |  |  |  |  |  | $ 1,200 |  |  |  |  |  |  |
BAS [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,200
Northeast Retirement Services, Inc. [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Purchase price of acquisition |  |  |  |  |  |  | $ 148,556 |  |  |  |  |  |  |  |
Cash |  |  |  |  |  |  | 70,073 |  |  |  |  |  |  |  |
Other liabilities |  |  |  |  |  |  | 26,828 |  |  |  |  |  |  |  |
Goodwill |  |  |  |  |  |  | 75,256 |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  | $ 23,000 |  |  |  |  |  |  |  |
Revenues earned |  |  |  |  |  |  |  |  | 9,700 |  | 8,400 | 19,800 | 13,400 |  |
Expenses incurred |  |  |  |  |  |  |  |  | $ 6,000 |  | $ 5,700 | $ 12,000 | $ 9,300 |  |
Increase (decrease) in other liabilities as a result of reclassification and adjustment |  |  |  |  |  |  |  |  |  | $ (1,200) |  |  |  |  |
Increase (decrease) in goodwill as a result of fair value adjustment |  |  |  |  |  |  |  |  |  | $ (1,200) |  |  |  |  |
Northeast Retirement Services, Inc. [Member] | Other Intangibles [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Acquisitions [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Intangibles |  |  |  |  |  |  | $ 60,200 |  |  |  |  |  |  |  |

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