Document and Entity Information (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended
Sep. 30, 2014
Oct. 31, 2014
Mar. 31, 2014
Document and Entity Information [Abstract]      
Entity Registrant Name WGL Holdings Inc.    
Entity Central Index Key 0001103601    
Document Type 10-K    
Document Period End Date Sep. 30, 2014    
Amendment Flag false    
Document Fiscal Year Focus 2014    
Document Fiscal Period Focus Q4    
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     $ 2,064,823,797
Entity Common Stock, Shares Outstanding   49,708,750