v3.26.1
Postretirement Benefit Plans (Pension Plan Assets) (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Mar. 31, 2025
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets $ 1,191 $ 1,257
Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 112 95
Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 57 55
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 160 140
Fixed Income [Member] | Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 104 86
Fixed Income [Member] | Level 2 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 56 54
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 2 3
Cash and Cash Equivalents | Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 2 3
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 1,022 1,107
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 7
Other [Member] | Level 1 [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Pension plan assets 6 6
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