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Fair Value Measurements (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Financial Instruments
Cash and cash equivalents	$ 472	$ 586	$ 621	$ 715
Institutional and Other | Minimum
Financial Instruments
Fair value assumption, interest rate	3.88%	3.78%
Institutional and Other | Maximum
Financial Instruments
Fair value assumption, interest rate	6.55%	6.93%
Fair Value | Level 1
Financial Instruments
Cash and cash equivalents	$ 472	$ 586
Fair Value | Level 2 | Retail
Financial Instruments
Long-term debt	966	929
Fair Value | Level 2 | Institutional
Financial Instruments
Long-term debt	552	532
Fair Value | Level 2 | Other
Financial Instruments
Long-term debt	197	203
Book Value
Financial Instruments
Cash and cash equivalents	472	586
Book Value | Retail
Financial Instruments
Long-term debt	917	917
Book Value | Institutional
Financial Instruments
Long-term debt	533	533
Book Value | Other
Financial Instruments
Long-term debt	$ 197	$ 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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82887183&loc=d3e6676-107765

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
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-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3044-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Topic 210
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Interest rate assumption used in valuing an instrument.

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