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Quarterly Financial Data (Unaudited) Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Statement [Abstract]          
Impairment losses     $ 431 $ 1,039 $ 1,040
Impairment of investments in unconsolidated affiliates $ 313 $ 308 $ 0 $ 313 $ 308