Recent Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Apr. 01, 2018	Mar. 31, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	$ 136		$ 136			$ 1,301
Income taxes payable	94		94			127
Deferred revenue and advance contract payments	1,442		1,442			$ 1,641
Pension Plans, Defined Benefit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service cost	22	$ 34	45	$ 66
Pension Plans, Defined Benefit | Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Service cost		$ 34		$ 66
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings	140		140		$ 114
Accrued sales commission					158
Income taxes payable	13		13		(40)
Deferred revenue and advance contract payments	$ (78)		$ (78)		(381)
Other current assets					$ (385)