Income Taxes - Tax Years Subject to Examination by Major Jurisdiction Table (Detail)	12 Months Ended
	Dec. 31, 2017
United States [Member] | Earliest Tax Year [Member]
Income TaxExamination [Line Items]
Tax year subject to examination	2012
United States [Member] | Latest Tax Year [Member]
Income TaxExamination [Line Items]
Tax year subject to examination	2017
Algeria [Member] | Earliest Tax Year [Member]
Income TaxExamination [Line Items]
Tax year subject to examination	2014
Algeria [Member] | Latest Tax Year [Member]
Income TaxExamination [Line Items]
Tax year subject to examination	2017
Ghana [Member] | Earliest Tax Year [Member]
Income TaxExamination [Line Items]
Tax year subject to examination	2014
Ghana [Member] | Latest Tax Year [Member]
Income TaxExamination [Line Items]
Tax year subject to examination	2017