v3.10.0.1
Document and Entity Information - USD ($)
$ in Thousands
12 Months Ended
Sep. 30, 2018
Oct. 31, 2018
Mar. 31, 2018
Document Information [Line Items]      
Entity Registrant Name Washington Gas Light Company    
Entity Central Index Key 0000104819    
Document Type 10-K    
Document Period End Date Sep. 30, 2018    
Amendment Flag false    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Current Fiscal Year End Date --09-30    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Non-accelerated Filer    
Entity Public Float     $ 0
Entity Common Stock, Shares Outstanding   46,479,536  
Entity Emerging Growth Company false    
Entity Small Business false    
Entity Shell Company false