Income Taxes (Schedule Of Income Tax Expense (Benefit)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current income tax expense (benefit):
United States federal, current income tax expense (benefit)	$ 5	$ (243)	$ 152
Various states, current income tax expense (benefit)	(11)	(8)	18
Canada and various provinces, current income tax expense (benefit)	106	14	307
Total current tax expense (benefit)	100	(237)	477
Deferred income tax expense (benefit):
United States federal, deferred income tax expense (benefit)	(3)	(5,033)	1,610
Various states, deferred income tax expense (benefit)		(336)	93
Canada and various provinces, deferred income tax expense (benefit)	(270)	(459)	188
Total deferred tax expense (benefit)	(273)	(5,828)	1,891
Total income tax expense (benefit)	$ (173)	$ (6,065)	$ 2,368