Document and Entity Information - shares
9 Months Ended
Sep. 30, 2019
Oct. 31, 2019
Document And Entity Information [Abstract]    
Entity Registrant Name MIDDLESEX WATER CO  
Entity Central Index Key 0000066004  
Document Type 10-Q  
Document Period End Date Sep. 30, 2019  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Entity Filer Category Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Common Stock, Shares Outstanding   16,669,540
Document Fiscal Period Focus Q3  
Document Fiscal Year Focus 2019  
Entity Shell Company false  
Entity Interactive Data Current Yes  
Entity Current Reporting Status Yes  
Entity File Number 0-422  
Entity Incorporation State or Country Code NJ  
Document Quarterly Report true  
Document Transition Report false