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Fair Value Measurements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financial Instruments
Cash and cash equivalents	$ 352	$ 586
Short-term investments	$ 50	$ 0
Retail | Minimum
Financial Instruments
Fair value assumption, interest rate	4.74%	3.78%
Retail | Maximum
Financial Instruments
Fair value assumption, interest rate	7.13%	6.93%
Fair Value | Level 1
Financial Instruments
Cash and cash equivalents	$ 352	$ 586
Short-term investments	50	0
Fair Value | Level 2 | Retail
Financial Instruments
Long-term debt	939	929
Fair Value | Level 2 | Institutional
Financial Instruments
Long-term debt	522	532
Fair Value | Level 2 | Other
Financial Instruments
Long-term debt	191	203
Book Value
Financial Instruments
Cash and cash equivalents	352	586
Short-term investments	50	0
Book Value | Retail
Financial Instruments
Long-term debt	917	917
Book Value | Institutional
Financial Instruments
Long-term debt	534	533
Book Value | Other
Financial Instruments
Long-term debt	$ 191	$ 203

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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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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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