Derivatives And Risk Management (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Derivative [Line Items]
Gas sales	$ 340	$ 458	$ 906	$ 1,540
Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative liabilities	8		8		$ 5
Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative liabilities	27		27
Not Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Derivative liabilities	8		8
Notional amount	170		$ 170
Derivative, expiration			Jun. 30, 2020
Designated as Hedging Instrument [Member] | Fixed Price Swaps [Member]
Derivative [Line Items]
Derivative gain (loss) in other comprehensive income		14		35
Gas sales		$ 50		$ 145
Natural Gas [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative liabilities	$ 42		$ 42