Derivative Instruments - Cash Flow Hedges (Details) $ in Millions		1 Months Ended	3 Months Ended		9 Months Ended
		Aug. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) Bcf	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Bcf	Sep. 30, 2018 USD ($)	Dec. 31, 2018 Bcf
Derivative [Line Items]
Natural gas held in storage (in cubic feet) | Bcf			5		5		5
Forecast sale of natural gas from storage designated as cash-flow hedges (in cubic feet) | Bcf			3		3		4
Unrealized loss included in other comprehensive income	[1]		$ (114)	$ (1)	$ (130)	$ (5)
Interest Rate Contract
Derivative [Line Items]
Unrealized loss included in other comprehensive income		$ 125
Interest Rate Contract | Senior Notes
Derivative [Line Items]
Debt issuance, face amount		$ 13,000

[1] Net of tax of $32 million and zero for the three months ended September 30, 2019, and 2018, and $36 million and $1 million for the nine months ended September 30, 2019, and 2018, respectively.