v3.25.1
Postretirement Benefit Plans (Pension Plan Assets) (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Mar. 31, 2024
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets $ 1,257 $ 1,276
Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 95 100
Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 55 56
Level 3 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 0 0
Fixed Income [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 140 147
Fixed Income [Member] | Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 86 92
Fixed Income [Member] | Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 54 55
Fixed Income [Member] | Level 3 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 0 0
Cash and Cash Equivalents    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 3 4
Cash and Cash Equivalents | Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 3 4
Cash and Cash Equivalents | Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 0 0
Cash and Cash Equivalents | Level 3 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 0 0
Investments measured at net asset value(1)    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 1,107 1,120
Investments measured at net asset value(1) | Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 0 0
Investments measured at net asset value(1) | Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 0 0
Investments measured at net asset value(1) | Level 3 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 0 0
Other [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 7 5
Other [Member] | Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 6 4
Other [Member] | Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 1 1
Other [Member] | Level 3 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets $ 0 $ 0