Fair Value Measurement - FV Hierarchy (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative asset, gross	$ 4,657	$ 931
Commodity derivative liability, gross	4,570	944
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation investments	297	306
Commodity derivative asset, gross	4,657	931
Total assets	4,954	1,237
Commodity derivative liability, gross	4,570	944
Total liabilities	4,570	944
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Deferred compensation investments	297	306
Commodity derivative asset, gross	4,221	744
Total assets	4,518	1,050
Commodity derivative liability, gross	4,200	765
Total liabilities	4,200	765
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative asset, gross	361	177
Total assets	361	177
Commodity derivative liability, gross	354	172
Total liabilities	354	172
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Commodity derivative asset, gross	75	10
Total assets	75	10
Commodity derivative liability, gross	16	7
Total liabilities	$ 16	$ 7