Fair Value Measurements, Including Derivatives (Schedule of Carrying Values and Estimated Fair Values of Debt Instruments) (Detail) - USD ($) $ in Millions		Jun. 30, 2019		Dec. 31, 2018
Fair Value Disclosures [Abstract]
Long-term debt, carrying amount	[1]	$ 4,171		$ 5,146
Long-term debt, estimated fair value	[1]	$ 4,982	[2]	$ 5,470	[3]

[1] Carrying amount includes amounts which represent the unamortized debt issuance costs and discount or premium.

[2] (1) Fair value is estimated using market prices, where available, and interest rates currently available for issuance of debt with similar terms and remaining maturities. All fair value measurements are classified as Level 2. The carrying amount of debt issuances with short-term maturities and variable rates refinanced at current market rates is a reasonable estimate of their fair value.

[3] (2) Fair value is estimated based on net present value calculations using independently sourced market data that incorporate a developed discount rate using similarly rated long-term debt, along with benchmark interest rates. All fair value measurements are classified as Level 2. The carrying amount of debt issuances with short-term maturities and variable rates refinanced at current market rates is a reasonable estimate of their fair value.