ACQUISITION - Consideration Paid and Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands		12 Months Ended
	Jan. 13, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Recognized amounts of identifiable assets acquired and liabilities assumed, at fair value:
Goodwill		$ 100,085	$ 4,935	$ 4,935
Loans		2,485,613	1,129,064
Fair value adjustment accretable yield		3,509	$ 0	$ 0
Lake Sunapee
Consideration paid:
Bar Harbor Bankshares common stock issued to Lake Sunapee Bank Group stockholders (4,163,853 shares)		181,919
Common stock issued to acquiree (in shares)	4,163,853
Cash paid for fractional shares		27
Total consideration paid	$ 181,920	181,946
Recognized amounts of identifiable assets acquired and liabilities assumed, at fair value:
Cash and short-term investments		39,564
Investment securities		155,579
Loans		1,208,199
Premises and equipment		22,210
Core deposit intangible		7,786
Other assets		51,879
Deposits		(1,150,611)
Borrowings		(232,277)
Deferred taxes, net		(8,466)
Other liabilities		(23,999)
Total identifiable net assets		86,796
Goodwill		95,150
Loans		1,200,000
Loans receivable, fair value disclosure		1,188,000
Fair value adjustment accretable yield		$ 1,090
Fair value discount, amortization period		3 years
Loans and leases receivable, deferred income		$ 11,400
Deposits acquired, amortization period		1 year
Borrowings acquired, weighted average contractual life		4 years
Lake Sunapee | Core Deposits
Recognized amounts of identifiable assets acquired and liabilities assumed, at fair value:
Acquired intangible assets, weighted average useful life		12 years
Lake Sunapee | Receivables Acquired with Deteriorated Credit Quality
Recognized amounts of identifiable assets acquired and liabilities assumed, at fair value:
Loans		$ 23,340
Loans receivable, fair value disclosure		18,450
Lake Sunapee | As Acquired
Recognized amounts of identifiable assets acquired and liabilities assumed, at fair value:
Cash and short-term investments	40,970
Investment securities	156,960
Loans	1,217,927
Premises and equipment	22,561
Core deposit intangible	0
Other assets	102,298
Deposits	(1,149,865)
Borrowings	(232,261)
Deferred taxes, net	(1,921)
Other liabilities	(19,912)
Total identifiable net assets	$ 136,757
Lake Sunapee | Fair Value Adjustments
Recognized amounts of identifiable assets acquired and liabilities assumed, at fair value:
Cash and short-term investments		(1,406)
Investment securities		(1,381)
Loans		(9,728)
Premises and equipment		(351)
Core deposit intangible		7,786
Other assets		(50,419)
Deposits		(746)
Borrowings		(16)
Deferred taxes, net		10,387
Other liabilities		(4,087)
Total identifiable net assets		$ (49,961)