Investments In and Advances To Affiliates - Summarized Financial Information (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 Equity Method Investments [Line Items]
Revenue	$ 3,544	$ 3,542	$ 3,229	$ 3,271	$ 3,219	$ 3,522	$ 3,656	$ 3,758	$ 13,586	$ 14,155	$ 16,124
Operating margin	662	688	574	509	717	665	755	721	2,433	2,858	2,980
Net income (loss)	(693)	$ 229	$ (387)	$ 74	41	$ 203	$ 264	$ 254	(777)	762	1,147
Assets, Current	6,411				6,842				6,411	6,842
Liabilities, Current	5,272				6,926				5,272	6,926
Liabilities, Noncurrent	24,365				22,835				24,365	22,835
Stockholders' Equity Attributable to Noncontrolling Interest	2,906				3,022				2,906	3,022
Undistributed Earnings Of Minority Owned Affiliates Included In Retained Earnings	246								246
Distributions Received From Minority Owned Affiliates									24	18	28
Basis Difference Between Carrying Amount And Investment	162								162
Minority Owned Affiliates
Schedule of Equity Method Investments [Line Items]
Revenue									586	641	928
Operating margin									145	152	206
Net income (loss)									64	210	59
Assets, Current	308				376				308	376
Assets, Noncurrent	2,577				2,132				2,577	2,132
Liabilities, Current	626				435				626	435
Liabilities, Noncurrent	1,209				1,044				1,209	1,044
Stockholders' Equity Attributable to Parent	1,048				1,029				1,048	1,029
Majority Owned Affiliates
Schedule of Equity Method Investments [Line Items]
Revenue									23	24	2
Operating margin									9	11	0
Net income (loss)									(2)	6	$ 0
Assets, Current	16				20				16	20
Assets, Noncurrent	181				211				181	211
Liabilities, Current	10				21				10	21
Liabilities, Noncurrent	122				153				122	153
Stockholders' Equity Attributable to Parent	$ 65				$ 57				$ 65	$ 57