Quarterly Financial Data - Unaudited (Details) - USD ($) $ 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
schedule of quarterly financial information Unaudited [Line Items]
Revenues	$ 5,310	$ 5,167	$ 4,585	$ 4,767	$ 5,396	$ 5,600	$ 5,369	$ 5,602	$ 19,829	$ 21,967	$ 23,239
Operating Income (Loss)	1,813	1,880	1,487	1,505	1,972	2,089	1,769	1,894	6,685	7,724	7,013
Net income	$ 993	$ 1,020	$ 772	$ 784	$ 1,084	$ 1,156	$ 963	$ 1,045	$ 3,569	$ 4,248	$ 3,869