v3.8.0.1
Document and Entity Information - shares
3 Months Ended
Dec. 31, 2017
Jan. 31, 2018
Document Information [Line Items]    
Entity Registrant Name WGL Holdings Inc.  
Entity Central Index Key 0001103601  
Document Type 10-Q  
Document Period End Date Dec. 31, 2017  
Amendment Flag false  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q1  
Current Fiscal Year End Date --09-30  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   51,359,182
Washington Gas Light Company    
Document Information [Line Items]    
Entity Registrant Name Washington Gas Light Company  
Entity Central Index Key 0000104819  
Document Type 10-Q  
Document Period End Date Dec. 31, 2017  
Amendment Flag false  
Document Fiscal Year Focus 2018  
Document Fiscal Period Focus Q1  
Current Fiscal Year End Date --09-30  
Entity Filer Category Non-accelerated Filer  
Entity Common Stock, Shares Outstanding   46,479,536