ACQUISITIONS (Details) shares in Thousands, $ in Thousands								3 Months Ended
	Jan. 02, 2018 USD ($)	Dec. 04, 2017 USD ($) shares	Nov. 17, 2017 USD ($)	May 12, 2017 USD ($) Location Branch shares	Mar. 01, 2017 USD ($)	Feb. 03, 2017 USD ($)	Jan. 02, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Cash								$ 1,464	$ 63,517
Goodwill								733,625	542,986	$ 734,430
Penna & Associates Agency, Inc. [Member]
Business Acquisition [Line Items]
Cash	$ 800
Goodwill	400
Penna & Associates Agency, Inc. [Member] | Customer Lists [Member]
Business Acquisition [Line Items]
Intangibles	400
Styles Bridges Associates [Member]
Business Acquisition [Line Items]
Cash	700
Styles Bridges Associates [Member] | Customer Lists [Member]
Business Acquisition [Line Items]
Intangibles	$ 700
Gordon B. Roberts Agency, Inc [Member]
Business Acquisition [Line Items]
Purchase price of acquisition		$ 3,700
Cash		$ 1,350
Issuance of common shares (in shares) | shares		40
Other liabilities		$ 700
Goodwill		2,200
Assets acquired		600
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] | Customer Lists [Member]
Business Acquisition [Line Items]
Intangibles			$ 1,200
Merchants Bancshares, Inc. [Member]
Business Acquisition [Line Items]
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
Expenses incurred								7,800
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 | Location				1
Merchants Bancshares, Inc. [Member] | Other Intangibles [Member]
Business Acquisition [Line Items]
Intangibles				$ 2,857
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
Northeast Retirement Services, Inc. [Member]
Business Acquisition [Line Items]
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								10,100	5,100
Expenses incurred								6,000	$ 3,600
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]
Business Acquisition [Line Items]
Intangibles						$ 60,200