Contingencies (Details) $ in Millions			1 Months Ended	2 Months Ended	4 Months Ended	6 Months Ended	12 Months Ended
	Feb. 01, 2018 MMBTU / d	Jan. 31, 2018 MMBTU / d	Jul. 31, 2017 MMBTU / d	Jan. 31, 2018 MMBTU / d	Nov. 30, 2017 MMBTU / d	Jun. 30, 2017 MMBTU / d	Dec. 31, 2018 USD ($) MMBTU / d contract lawsuit	Dec. 31, 2016 MMBTU / d	Nov. 30, 2018 MMBTU / d	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
SJGC
Contingencies
Natural gas long term purchase contract volume (in MMBtu)/day							80,000
WGL
Contingencies
Natural gas long term purchase contract volume after specified events (in MMBtu)/day									530,000
WGL | Pending Litigation
Contingencies
Damages sought | $							$ 109			$ 40
WGL | Pending Litigation | Minimum
Contingencies
Damages sought | $											$ 30
WGL - Braxton, West Virginia
Contingencies
Natural gas long term purchase contract volume (in MMBtu)/day	200,000	500,000
WGL - Loudoun County, Virginia
Contingencies
Natural gas long term purchase contract volume increase after specified events (in MMBtu)/day									330,000
Potential Positive Outcome of Litigation | SJGC
Contingencies
Number of lawsuits | lawsuit							2
Number of long term gas contracts | contract							2
Damages sought | $							$ 86
Potential Positive Outcome of Litigation | WGL
Contingencies
Natural gas long term purchase contract volume (in MMBtu)/day			500,000	500,000	600,000	500,000		500,000