Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Deferred tax assets, property and equipment	$ 685	$ 490
Deferred tax assets, asset retirement obligations	488	485
Deferred tax assets, accrued liabilities	130	160
Deferred tax assets, net operating loss carryforwards	777	175
Deferred tax assets, pension benefit obligations	98	106
Deferred tax assets, other	203	162
Total deferred tax assets before valuation allowance	2,381	1,578
Less: valuation allowance	(1,666)	(967)
Net deferred tax assets	715	611
Deferred tax liabilities, property and equipment	(884)	(1,187)
Deferred tax liabilities, long-term debt	(53)	(36)
Deferred tax liabilities, other	(426)	(271)
Total deferred tax liabilities	(1,363)	(1,494)
Net deferred tax liability	$ (648)	$ (883)