Pension and Other Benefit Plans - Fair Value by Investment Category and Level Within Fair Value Hierarchy (Details) - Pension Plans, Defined Benefit - USD ($) $ in Millions	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 11,090	$ 11,343	$ 11,574
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	669	788
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9,063	9,523
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,358	1,032	$ 887
US Domestic Stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	1
US Domestic Stocks | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
US Domestic Stocks | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	0
US Domestic Stocks | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Global Stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	23
Global Stocks | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	10
Global Stocks | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	13
Global Stocks | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Global/International Equity commingled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,078	2,555
Global/International Equity commingled funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	315	399
Global/International Equity commingled funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,763	2,156
Global/International Equity commingled funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Global equity mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	374
Global equity mutual funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	49
Global equity mutual funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	325
Global equity mutual funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S./North American Equity commingled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5	11
U.S./North American Equity commingled funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	1
U.S./North American Equity commingled funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	10
U.S./North American Equity commingled funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Non-U.S. Government funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	136	244
Non-U.S. Government funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	136	215
Non-U.S. Government funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	29
Non-U.S. Government funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income commingled funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	126	4,813
Fixed income commingled funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55	6
Fixed income commingled funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	71	4,807
Fixed income commingled funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Fixed income mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	3
Fixed income mutual funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	2
Fixed income mutual funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Fixed income mutual funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,808	2
Corporate bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	0
Corporate bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,807	2
Corporate bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Alternatives
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,335	2,868
Other Alternatives | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	6
Other Alternatives | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,038	1,880
Other Alternatives | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,297	982
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	8
Hedge Funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	0
Hedge Funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7	8
Hedge Funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	0
Other Assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	375	36
Other Assets | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	87	0
Other Assets | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	229	0
Other Assets | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	59	36
Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	136	122
Insurance contracts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Insurance contracts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	136	108
Insurance contracts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	14
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63	283
Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	61	99
Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	184
Cash and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0