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Cover - USD ($)
 12 Months Ended
Dec. 31, 2020
Mar. 01, 2021
Jun. 30, 2020
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Entity File Number 0-26589    
Entity Registrant Name FIRST BANCORP, INC    
Entity Incorporation, State or Country Code ME    
Entity Tax Identification Number 01-0404322    
Entity Address, Address Line One 223 Main Street    
Entity Address, City or Town Damariscotta    
Entity Address, State or Province ME    
Entity Address, Postal Zip Code 04543    
Entity Central Index Key 0000765207    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Non-accelerated Filer    
Entity Public Float     $ 222,030,000
Entity Common Stock, Shares Outstanding   10,984,439  
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Emerging Growth Company false    
Entity Small Business true    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
City Area Code 207    
Local Phone Number 563-3195    
Document Transition Report false