Derivatives and Risk Management (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 item	Dec. 03, 2018 USD ($)
Derivative [Line Items]
Gain on sale of assets, net	$ (17)	$ (6)
Derivative assets	$ 51
Number of derivative positions designated for hedge accounting | item	0	0	0
Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative liabilities		$ 1
Derivative assets	$ 1
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Number of derivative positions designated for hedge accounting | item	0
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Notional amount	$ 170
Derivative, expiration	Jun. 30, 2020
Fayetteville Shale [Member]
Derivative [Line Items]
Derivative liabilities				$ 151
Gain on sale of assets, net	$ 22