Fair Value Measurement - FV Hierarchy (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in Cenovus Energy	$ 971	$ 2,111
Investments in debt securities	475	241
Commodity derivative asset, gross	351	322
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in Cenovus Energy	971	2,111
Investments in debt securities	475	241
Commodity derivative asset, gross	351	322
Total assets	1,797	2,674
Commodity derivative liability, gross	335	311
Total liabilities	335	311
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in Cenovus Energy	971	2,111
Investments in debt securities	26	25
Commodity derivative asset, gross	207	172
Total assets	1,204	2,308
Commodity derivative liability, gross	216	174
Total liabilities	216	174
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in Cenovus Energy	0	0
Investments in debt securities	449	216
Commodity derivative asset, gross	120	114
Total assets	569	330
Commodity derivative liability, gross	103	115
Total liabilities	103	115
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in Cenovus Energy	0	0
Investments in debt securities	0	0
Commodity derivative asset, gross	24	36
Total assets	24	36
Commodity derivative liability, gross	16	22
Total liabilities	$ 16	$ 22

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and elected not to be offset.

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Fair value as of the balance sheet date of derivative instrument not designated as hedging instrument and classified as a liability.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Fair value of financial and nonfinancial obligations.

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