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Summary of Significant Accounting Policies - Schedule of Change in Asset Retirement Obligation (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Asset Retirement Obligation [Line Items]
Beginning balance	$ 0
Acquisition of Credo	1,255
Accretion expense	26
Additions	79
Ending balance	$ 1,360

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.

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- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

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