| Purchase Price Allocation for Acquisition of Properties (Detail) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2013 | Dec. 31, 2012 | Oct. 05, 2012
 Newfield Properties | 
|---|---|---|---|
| Business Acquisition [Line Items] | |||
| Oil and natural gas properties and equipment | $ 237,396 | ||
| Asset retirement obligations – current | (74,687) | (92,630) | (7,250) | 
| Asset retirement obligations – non-current | (309,918) | (291,423) | (24,414) | 
| Total cash paid | $ 205,732 | 
| X | ||||||||||
| 
- Definition Current portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
 | 
| X | ||||||||||
| 
- Definition Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
 | 
| X | ||||||||||
| 
- Definition Amount of cash paid to acquire the entity. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
 | 
| X | ||||||||||
| 
- Details 
 | 
| X | ||||||||||
| 
- Definition The amount of acquisition cost of a business combination allocated to natural resources other than mineral rights in the mining industry. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
 |