Basis of Presentation Basis of Presentation and Summary of Significant Accounting Policies Accounts Receivable Table (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Oil, natural gas and NGL sales, receivables	$ 959	$ 840
Oil and Gas Joint Interest Billing Receivables, Current	209	156
Other Receivables	184	93
Allowance for Doubtful Accounts Receivable	(30)	(32)
Accounts Receivable, Net, Current	$ 1,322	$ 1,057

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For unclassified balance sheet, amounts due from customers or clients for goods or services that have been delivered or sold in the normal course of business.

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Amount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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For an unclassified balance sheet, a valuation allowance for receivables due a company that are expected to be uncollectible.

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Current portion of accounts receivable attributable to joint interest partners in oil and gas properties. These are billings to the non-operators associated with the operation of wells and are based on those owners' working interests in the wells. For classified balance sheets, represents the current amount receivable, that is amounts expected to be collected within one year or the normal operating cycle, if longer.

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Amount due from parties in nontrade transactions, classified as other.

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