Other Material Contingencies Other Material Contingencies - Manhattan Explosion and Fire (Details) $ in Millions		1 Months Ended
	Mar. 12, 2014 building people	Feb. 28, 2017 USD ($)	Sep. 30, 2017 USD ($) lawsuit	Dec. 31, 2016 USD ($)
Loss Contingencies [Line Items]
Accrued regulatory liability			$ 1,873	$ 1,905
Settlement of Gas Proceedings
Loss Contingencies [Line Items]
Accrued regulatory liability			$ 27	$ 27
Manhattan Explosion and Fire
Loss Contingencies [Line Items]
Number of buildings destroyed by fire | building	2
Number of people died in explosion and fire incident | people	8
Number of people injured in explosion and fire incident (more than) | people	50
Amount of costs that will not recover from customers		$ 126
Number of pending lawsuits | lawsuit			80
Manhattan Explosion and Fire | Settlement of Gas Proceedings
Loss Contingencies [Line Items]
Accrued regulatory liability			$ 27