Pension and Other Benefit Plans - Pension Plan, Assumptions and Other Selected Information (Details) - Pension Plans, Defined Benefit - USD ($) $ in Millions	Mar. 31, 2019	Mar. 31, 2018
Benefit Plans with Projected Benefit Obligation in Excess of Plan Assets
Projected benefit obligation	$ 2,329	$ 2,488
Accumulated benefit obligation	2,230	2,363
Fair value of plan assets	1,494	1,552
Benefit Plans with Accumulated Benefit Obligation in Excess of Plan Assets
Projected benefit obligation	2,070	2,250
Accumulated benefit obligation	2,004	2,162
Fair value of plan assets	$ 1,255	$ 1,338