Quarterly Financial And Market Information (Summary Of Quarterly Financial And Market Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended														12 Months Ended
	Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019		Mar. 31, 2018	Mar. 31, 2017
Quarterly Financial And Market Information [Line Items]
Income Tax Expense (Benefit)															$ 60		$ 406	$ 243
Net revenue	$ 1,238	[1]	$ 1,289	[1]	$ 1,286	[1]	$ 1,137	[1]	$ 1,582		$ 1,160		$ 959	$ 1,449	4,950	[1]	5,150	4,845
Gross profit	962		876		868		922		1,349		659		570	1,295	3,628		3,873	3,547
Operating income (loss)	196		242		258		300		753		(21)		(41)	743	996		1,434	1,224
Net income (loss)	$ 209		$ 262		$ 255		$ 293		$ 607	[2]	$ (186)	[2]	$ (22)	$ 644	$ 1,019		$ 1,043	$ 967
Earnings Per Share, Basic	$ 0.70		$ 0.87		$ 0.84		$ 0.96		$ 1.98		$ (0.60)		$ (0.07)	$ 2.08	$ 3.36		$ 3.39	$ 3.19
Earnings Per Share, Diluted	$ 0.69		$ 0.86		$ 0.83		$ 0.95		$ 1.95		$ (0.60)		$ (0.07)	$ 2.06	$ 3.33		$ 3.34	$ 3.08
Total U.S. Tax Cuts and Jobs Act impact [Member]
Quarterly Financial And Market Information [Line Items]
Income Tax Expense (Benefit)									$ 59		$ 176						$ 235

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[2] During the quarters ended December 31, 2017 and March 31, 2018, we recognized tax expense of $176 million and $59 million, respectively, due to the application of the U.S. Tax Act, enacted on December 22, 2017.