Fair Value Measurements - Financial Instruments (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Financial Instruments
Cash and cash equivalents	$ 285	$ 580
Short-term investments	0	17
Book Value
Financial Instruments
Cash and cash equivalents	285	580
Short-term investments	0	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	83	180
Fair Value \| Level 1
Financial Instruments
Cash and cash equivalents	285	580
Short-term investments	0	17
Fair Value \| Level 2 \| Retail
Financial Instruments
Long-term debt	943	850
Fair Value \| Level 2 \| Institutional
Financial Instruments
Long-term debt	594	531
Fair Value \| Level 2 \| Other
Financial Instruments
Long-term debt	$ 83	$ 180
7.25% 2063 Senior Notes
Financial Instruments
Interest rate on debt	7.25%
7.25% 2064 Senior Notes
Financial Instruments
Interest rate on debt	7.25%
6.95% Senior Notes
Financial Instruments
Interest rate on debt	6.95%
6.7% Senior Notes
Financial Instruments
Interest rate on debt	6.70%
Interest rate \| Minimum \| Institutional and Other
Financial Instruments
Fair value assumption, interest rate	3.55%	5.03%
Interest rate \| Maximum \| Institutional and Other
Financial Instruments
Fair value assumption, interest rate	5.73%	6.97%

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- Definition

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.

+ References

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- Definition

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

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- Definition

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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- Definition

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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