Income Taxes (Schedule Of Deferred Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Deferred tax assets, asset retirement obligations	$ 313	$ 488
Deferred tax assets, accrued liabilities	62	130
Deferred tax assets, net operating loss carryforwards	865	777
Deferred tax assets, pension benefit obligations	54	98
Deferred tax assets, Canadian capital loss carryforward	760	17
Deferred tax assets, other	135	186
Total deferred tax assets before valuation allowance	2,189	1,696
Less: valuation allowance	(968)	(645)	$ (403)
Net deferred tax assets	1,221	1,051
Deferred tax liabilities, property and equipment	(1,703)	(1,635)
Deferred tax liabilities, long-term debt	(92)	(53)
Deferred tax liabilities, other	(261)	(426)
Total deferred tax liabilities	(2,056)	(2,114)
Net deferred tax liability	$ (835)	$ (1,063)