Debt (Summary of Long-Term Debt) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Debt at face value	$ 3,550	$ 3,550
Net unamortized discounts and debt issuance costs	(34)	(34)
Total debt	3,516	3,516
Short-term debt	(25)	(25)
Long-term debt	3,491	3,491
Tax-exempt bonds due April 2020 and April 2021 at 1.465% and 1.85% at March 31, 2020, and December 31, 2019, respectively
Debt Instrument [Line Items]
Long-term debt, gross	$ 75	$ 75
Interest rate, stated percentage	1.465%	1.85%
Senior Notes \| 2.450% Senior Notes due December 2024
Debt Instrument [Line Items]
Long-term debt, gross	$ 300	$ 300
Interest rate, stated percentage	2.45%
Senior Notes \| 3.605% Senior Notes due February 2025
Debt Instrument [Line Items]
Long-term debt, gross	$ 500	500
Interest rate, stated percentage	3.605%
Senior Notes \| 3.550% Senior Notes due October 2026
Debt Instrument [Line Items]
Long-term debt, gross	$ 500	500
Interest rate, stated percentage	3.55%
Senior Notes \| 3.750% Senior Notes due March 2028
Debt Instrument [Line Items]
Long-term debt, gross	$ 500	500
Interest rate, stated percentage	3.75%
Senior Notes \| 3.150% Senior Notes due December 2029
Debt Instrument [Line Items]
Long-term debt, gross	$ 600	600
Interest rate, stated percentage	3.15%
Senior Notes \| 4.680% Senior Notes due February 2045
Debt Instrument [Line Items]
Long-term debt, gross	$ 450	450
Interest rate, stated percentage	4.68%
Senior Notes \| 4.900% Senior Notes due October 2046
Debt Instrument [Line Items]
Long-term debt, gross	$ 625	$ 625
Interest rate, stated percentage	4.90%

Amount of short-term and long-term debt and lease obligation.

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