Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Financial Instruments
Cash and cash equivalents	$ 528	$ 580
Short-term investments	$ 18	17
7.25% 2063 Senior Notes
Financial Instruments
Interest rate	7.25%
7.25% 2064 Senior Notes
Financial Instruments
Interest rate	7.25%
6.95% Senior Notes
Financial Instruments
Interest rate	6.95%
6.7% Senior Notes
Financial Instruments
Interest rate	6.70%
Book Value
Financial Instruments
Cash and cash equivalents	$ 528	580
Short-term investments	18	17
Book Value \| Retail
Financial Instruments
Long-term debt	917	917
Book Value \| Institutional
Financial Instruments
Long-term debt	534	534
Book Value \| Other
Financial Instruments
Long-term debt	174	180
Fair Value \| Level 1
Financial Instruments
Cash and cash equivalents	528	580
Short-term investments	18	17
Fair Value \| Level 2 \| Retail
Financial Instruments
Long-term debt	948	850
Fair Value \| Level 2 \| Institutional
Financial Instruments
Long-term debt	578	531
Fair Value \| Level 2 \| Other
Financial Instruments
Long-term debt	$ 174	$ 180
Interest rate \| Institutional and Other \| Minimum
Financial Instruments
Fair value assumption, interest rate	4.16%	5.03%
Interest rate \| Institutional and Other \| Maximum
Financial Instruments
Fair value assumption, interest rate	6.05%	6.97%

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Amount of investments including trading securities, available-for-sale securities, held-to-maturity securities, and short-term investments classified as other and current.

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Interest rate assumption used in valuing an instrument.

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