Revenues (Tables)

6 Months Ended

Jun. 30, 2018

Revenue Recognition and Deferred Revenue [Abstract]

Revenue segment information

Country	Six-month period ended June 30,
	2017		2018
Norway	$	27,161	$	15,918
Brazil		171,259		106,612
Tanzania		-		9,164
Angola		288,957		119,693
Congo		99,940		40,106
Total service revenues	$	587,317	$	291,493

Contract Liabilities

	December 31, 2017		June 30, 2018
Contract liabilities – current		15,329		5,230
Contract liabilities – non-current	$	14,385	$	11,792

Changes in contract liabilities

	Contract Liabilities
Balance as of December 31, 2017	$	29,714
Decrease: Amortization of deferred revenue		13,428
Addition: Transfer to Trade accounts receivable		736
Balance as of June 30, 2018	$	17,022

Future Amortization of Contract Liabilities and Deferred Costs

	Remaining 2018		2019		2020		2021		Total
Amortization of contract liabilities	$	2,637	$	5,229	$	5,244	$	3,912	$	17,022
Amortization of deferred costs	$	748	$	1,483	$	1,486	$	1,110	$	4,827