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Quarterly Financial Data (Unaudited) Narrative (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
[1]
Dec. 31, 2015
[1]
Income Statement [Abstract]      
Impairment losses $ 1,039 $ 1,040 $ 339
Impairment of investments in unconsolidated affiliates $ 313 $ 308 $ 0
[1] * As adjusted. See Note 2.