Derivatives and Risk Management (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($) item	Dec. 31, 2015 USD ($) item
Derivative [Line Items]
Number of derivative position designated for hedge accounting | item	0	0	0
Gas sales	$ 1,793	$ 1,273	$ 1,946
Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative liabilities	1	3
Notional amount	$ 170
Derivative, expiration	Jun. 30, 2020
Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative assets	$ 56	1
Derivative liabilities	$ 1	$ 178
Designated as Hedging Instrument [Member]
Derivative [Line Items]
Number of derivative position designated for hedge accounting | item	0	0
Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member]
Derivative [Line Items]
Gas sales			209
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Number of derivative position designated for hedge accounting | item	0
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative assets	$ 77
Natural Gas [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative liabilities	$ 1
Mark To Market Gain (Loss) On Derivatives [Member] | Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative gain (loss) in other comprehensive income			$ 45