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, 2021	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 13,425	$ 11,090	$ 11,343
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	419	669
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,975	9,063
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,031	1,358	$ 1,032
US Domestic Stocks
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3
US Domestic Stocks | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
US Domestic Stocks | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3
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	0	3
Global Stocks | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Global Stocks | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3
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	2,506	2,078
Global/International Equity commingled funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	246	315
Global/International Equity commingled funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,260	1,763
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	0	8
Global equity mutual funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	8
Global equity mutual funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
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	6	5
U.S./North American Equity commingled funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
U.S./North American Equity commingled funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6	4
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	0	136
Non-U.S. Government funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	136
Non-U.S. Government funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
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	58	126
Fixed income commingled funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	55
Fixed income commingled funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	42	71
Fixed income commingled funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	0
Fixed income mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	3
Fixed income mutual funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	3
Fixed income mutual funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	0
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	5,500	4,808
Corporate bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	1
Corporate bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,500	4,807
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	4,637	3,335
Other Alternatives | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	0
Other Alternatives | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,706	2,038
Other Alternatives | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,930	1,297
Hedge Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	11
Hedge Funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	2
Hedge Funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	7
Hedge Funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	2
Other Assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	220	375
Other Assets | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	70	87
Other Assets | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	65	229
Other Assets | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	85	59
Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	384	136
Insurance contracts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4	0
Insurance contracts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	380	136
Insurance contracts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	99	63
Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	97	61
Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	2
Cash and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0