v3.8.0.1
Document and Entity Information - USD ($)
12 Months Ended
Sep. 30, 2017
Oct. 31, 2017
Mar. 31, 2017
Document Information [Line Items]      
Entity Registrant Name WGL Holdings Inc.    
Entity Central Index Key 0001103601    
Document Type 10-K    
Document Period End Date Sep. 30, 2017    
Amendment Flag false    
Document Fiscal Year Focus 2017    
Document Fiscal Period Focus FY    
Current Fiscal Year End Date --09-30    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Public Float     $ 4,187,038,726
Entity Common Stock, Shares Outstanding   51,352,540  
Washington Gas Light Company      
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, 2017    
Amendment Flag false    
Document Fiscal Year Focus 2017    
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