Segment Information 1 (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Summary of segment revenue and reconciliation to total consolidated revenue
Segment revenue	$ 1,045		$ 1,034	$ 2,097		$ 2,059
Impact of segment policies that differ from U.S. GAAP	(150)	[1]	0	(264)	[1]	0
Total consolidated revenue	895		1,034	1,833		2,059
Mainframe Solutions
Summary of segment revenue and reconciliation to total consolidated revenue
Segment revenue	548		539	1,101		1,075
Total consolidated revenue	388			828
Enterprise Solutions
Summary of segment revenue and reconciliation to total consolidated revenue
Segment revenue	423		420	848		834
Total consolidated revenue	436			866
Services
Summary of segment revenue and reconciliation to total consolidated revenue
Segment revenue	74		$ 75	148		$ 150
Total consolidated revenue	$ 71			$ 139

[1] The manner in which the Company measures and recognizes revenues for segment reporting was not revised upon adoption of Topic 606. For segment reporting purposes, the Company follows its previous Topic 605 policies, which recognizes software license revenue ratably, except for sales of perpetual licenses on a stand-alone basis, which are recognized at a point-in-time.