Fair Value Measurements - Assets and Liabilities Accounted for at Fair Value on Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash collateral netted with derivative liabilities	$ 47		$ 32
Liabilities, current	370		159
Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivative instruments, assets - netting and collateral	(44)	[1]	(21)	[1]
Commodity derivative instruments, assets - net	0	[2]	0	[2]
Commodity derivative instruments, assets - collateral pledged not offset	60		61
Other assets	2		2
Total assets at fair value	2		2
Commodity derivative instruments, liabilities - netting and collateral	91	[1]	53	[1]
Commodity derivative instruments, liabilities - net	0	[2]	0	[2]
Commodity derivative instruments, liabilities - collateral pledged not offset	0		0
Contingent consideration, liability	647	[3]	625	[3]
Total liabilities at fair value	647		625
Level 1 | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivative instruments, assets - gross	44		21
Other assets	2		2
Total assets at fair value	46		23
Commodity derivative instruments, liabilities - gross	91		53
Contingent consideration, liability	0		0
Total liabilities at fair value	91		53
Level 2 | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivative instruments, assets - gross	0		0
Other assets	0		0
Total assets at fair value	0		0
Commodity derivative instruments, liabilities - gross	0		0
Contingent consideration, liability	0		0
Total liabilities at fair value	0		0
Level 3 | Fair Value, Measurements, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Commodity derivative instruments, assets - gross	0		0
Other assets	0		0
Total assets at fair value	0		0
Commodity derivative instruments, liabilities - gross	0		0
Contingent consideration, liability	647	[3]	625	[3]
Total liabilities at fair value	$ 647		$ 625

[1] Represents the impact of netting assets, liabilities and cash collateral when a legal right of offset exists. As of June 30, 2014 and December 31, 2013, cash collateral of $47 million and $32 million, respectively, was netted with mark-to-market derivative liabilities.

[2] We have no derivative contracts that are subject to master netting arrangements that are reflected gross on the balance sheet.

[3] Includes $370 million and $159 million classified as current at June 30, 2014 and December 31, 2013, respectively.