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Revenues from Contracts with Customers Revenues from Contracts with Customers - Contract Liabilities (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Revenue from Contract with Customer [Abstract]              
Contract with Customer, Liability, beginning balance     $ 1,060 $ 1,087 $ 1,087    
Deferred Revenue, Additions     220   377    
Contract with Customer, Liability, Revenue Recognized $ (78) $ (56) (135) $ (116) 229 $ 132 $ 211
Reclassification from deferred revenues due to change in estimated transaction price     (92)   175    
Contract with Customer, Liability, ending balance $ 1,053   $ 1,053   $ 1,060 $ 1,087