Pension and Other Benefit Plans - Fair Value by Investment Category and Level Within Fair Value Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2020	Mar. 31, 2019	May 31, 2018	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	$ 155	$ 247	$ 419
Total investments measured at NAV	214	154
Total investments	369	401
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	46	18
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	55	151
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	54	78		$ 0
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	6	12
Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	0	0
Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	6	12
Cash and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	0	0
Equity funds
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	60	122
Total investments measured at NAV	116	20
Equity funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	17	18
Equity funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	43	104
Equity funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	0	0
Fixed income
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	35	35
Total investments measured at NAV	15	32
Fixed income | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	29	0
Fixed income | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	6	35
Fixed income | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	0	0
Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	46	53
Total investments measured at NAV	83	102
Real estate funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	0	0
Real estate funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	0	0
Real estate funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	46	53		0
Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	8	25
Hedge funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	0	0
Hedge funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	0	0
Hedge funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total investments measured at fair value	$ 8	$ 25		$ 0