Commitments and Contingencies - Litigation (Details) - Court of Ouargla Foreign Currency Controls - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2017	Dec. 31, 2006
Commitments and Contingencies, Disclosure
Litigation amount as per judgment	$ 24.6
Payment of contract amount in foreign currency			$ 7.5
Payment of contract amount in domestic currency			$ 3.2
Maximum
Commitments and Contingencies, Disclosure
Potential judgment in excess of accrual		$ 16.6