Operating Revenue (Schedule of Aggregate Amount of Transaction Price Allocated To Fixed-price Performance Obligations That Unsatisfied At End of Reporting Period And Expected To be Recognized) (Detail) $ in Millions	Mar. 31, 2021 USD ($)
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 788
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2021-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 51
Revenue, expected to be recognized on multi-year contracts, period	9 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2022-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 68
Revenue, expected to be recognized on multi-year contracts, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2023-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 66
Revenue, expected to be recognized on multi-year contracts, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2024-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 59
Revenue, expected to be recognized on multi-year contracts, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2025-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 51
Revenue, expected to be recognized on multi-year contracts, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date: 2026-01-01
Revenues From Contract With Customer [Line Items]
Revenue, expected to be recognized on multi-year contracts	$ 493
Revenue, expected to be recognized on multi-year contracts, period