Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cumulative Translation Adjustment Summary [Roll Forward]
Beginning balance	$ (306)	$ (800)	$ (419)	$ (890)	$ (890)
Net gain/(loss) on Foreign Currency Transaction	(364)	191	(251)	281
Ending balance	$ (670)	$ (609)	$ (670)	$ (609)	(419)	$ (890)
Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax [Abstract]
Net gain/(loss) on foreign currency translation, tax adjustment					$ 0	$ 0	$ 0