Segment Information (Net Revenue By Geographic Area) (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Mar. 31, 2019	[1]	Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019		Mar. 31, 2018	Mar. 31, 2017
Revenue from External Customer [Line Items]
Net revenue	$ 1,238		$ 1,289		$ 1,286		$ 1,137		$ 1,582	$ 1,160	$ 959	$ 1,449	$ 4,950	[1]	$ 5,150	$ 4,845
North America [Member]
Revenue from External Customer [Line Items]
Net revenue													1,906		2,090	2,119
International [Member]
Revenue from External Customer [Line Items]
Net revenue													$ 3,044		$ 3,060	$ 2,726

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