Fair Value Measurement (Fair Value Hierarchy for Gross Financial Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in Cenovus Energy	$ 1,117	$ 1,256
Commodity derivatives	1,243	255
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investment in Cenovus Energy	1,117	1,256
Investments in debt securities	479	518
Commodity derivatives	1,243	255
Total assets	2,839	2,029
Commodity derivatives	1,223	220
Total liabilities	1,223	220
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	1,117	1,256
Investments in debt securities	2	17
Commodity derivatives	562	142
Total assets	1,681	1,415
Commodity derivatives	593	120
Total liabilities	593	120
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	477	501
Commodity derivatives	619	101
Total assets	1,096	602
Commodity derivatives	543	91
Total liabilities	543	91
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 derivatives	62	12
Total assets	62	12
Commodity derivatives	87	9
Total liabilities	$ 87	$ 9