REVENUE FROM CONTRACTS WITH CUSTOMERS - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	$ 11,732	$ 4,646	$ 22,146	$ 13,564
Within the scope of ASC 606
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	9,297	7,852	17,521	15,293
Not within the scope of ASC 606
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	2,435	(3,206)	4,625	(1,729)
Products and services transferred at a point in time \| Within the scope of ASC 606
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	4,428	3,813	8,584	7,622
Products and services transferred over time \| Within the scope of ASC 606
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	4,869	4,039	8,937	7,671
Location, Statement of Income, Balance [Axis]: bhb:NonInterestIncomeMember \| Within the scope of ASC 606
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	9,297	7,852	17,521	15,293
Trust management fees \| Within the scope of ASC 606
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	4,132	3,865	7,808	7,324
Financial services fees \| Within the scope of ASC 606
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	445	398	885	855
Interchange fees \| Within the scope of ASC 606
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	2,028	1,850	3,939	3,771
Customer deposit fees \| Within the scope of ASC 606
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	1,887	1,470	3,643	2,847
Other customer service fees \| Within the scope of ASC 606
REVENUE FROM CONTRACTS WITH CUSTOMERS
Total non-interest income	$ 805	$ 269	$ 1,246	$ 496