Document and Entity Information (USD $)
In Thousands, except Share data, unless otherwise specified
12 Months Ended
Sep. 30, 2011
Jan. 29, 2010
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, 2011  
Amendment Flag false  
Document Fiscal Year Focus 2011  
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 $ 0  
Entity Common Stock, Shares Outstanding   51,429,169