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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2020
Feb. 25, 2021
Jun. 30, 2020
Document And Entity Information [Abstract]      
Document Type 10-K    
Entity Central Index Key 0000066004    
Document Period End Date Dec. 31, 2020    
Document Fiscal Period Focus FY    
Document Fiscal Year Focus 2020    
Amendment Flag false    
Document Annual Report true    
Document Transition Report false    
Entity File Number 0-422    
Entity Registrant Name MIDDLESEX WATER COMPANY    
Entity Incorporation State Country Code NJ    
Entity Tax Identification Number 22-1114430    
Entity Address, Address Line One 485C Route 1 South    
Entity Address, Address Line Two Suite 400    
Entity Address, City or Town Iselin    
Entity Address, State or Province NJ    
Entity Address, Postal Zip Code 08830    
City Area Code 732    
Local Phone Number 634-1500    
Title of 12(b) Security Common Stock, No Par Value    
Trading Symbol MSEX    
Name of Exchange on which Security is Registered NASDAQ    
Current Fiscal Year End Date --12-31    
Entity a Well-known Seasoned Issuer Yes    
Entity a Voluntary Filer No    
Entity Reporting Status Current Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 1,152,416,738
Entity Common Stock, Shares Outstanding   17,474,598  
Documents Incorporated By Reference Text Block

Proxy Statement to be filed in connection with the Registrant’s Annual Meeting of Stockholders to be held on May 25, 2021, which will be filed with the Securities and Exchange Commission within 120 days of the end of our 2020 fiscal year, is incorporated by reference into Part III of this Annual Report on Form 10-K to the extent described herein.