Derivatives And Risk Management (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Gas sales	$ 1,273	$ 1,946	$ 2,827
Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative liabilities	3	5
Notional amount	$ 170
Derivative, expiration	Jun. 30, 2020
Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative liabilities	$ 178
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative liabilities	3
Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member]
Derivative [Line Items]
Gas sales		209
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative liabilities	$ 372
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