Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2018
Feb. 28, 2019
Jun. 30, 2018
Document And Entity Information [Abstract]      
Entity Registrant Name MIDDLESEX WATER CO    
Entity Central Index Key 0000066004    
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Entity a Well-known Seasoned Issuer No    
Entity a Voluntary Filer No    
Entity Reporting Status Current Yes    
Entity Filer Category Accelerated Filer    
Entity Small Business false    
Entity Shell Company false    
Entity Emerging Growth Company false    
Entity Public Float     $ 664,121,962
Entity Common Stock, Shares Outstanding   16,429,308  
Document Fiscal Period Focus FY    
Document Fiscal Year Focus 2018