Supplemental Quarterly Financial Information (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Data [Abstract]
Asset impairments	$ 100	$ 300	$ 1,500	$ 3,000	$ 5,300	$ 5,900	$ 4,200	$ 5,500	$ 4,975	$ 20,820	$ 1,953
Asset impairment per diluted share	$ 0.24	$ 0.61	$ 2.89	$ 6.40	$ 13.09	$ 14.41	$ 10.27	$ 13.46
Asset dispositions and other	$ 540	$ 1,400							$ 1,893		$ 1,072
Gain on disposition of assets per diluted share	$ 1.04	$ 2.59