Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue by Geographic Area [Line Items]
Revenues	$ 478,674	$ 416,413	$ 515,036	$ 895,087	$ 961,212
Africa [Member]
Revenue by Geographic Area [Line Items]
Revenues	61,966	55,087	74,759	117,053	155,271
UNITED KINGDOM
Revenue by Geographic Area [Line Items]
Revenues	50,999	45,319	74,714	96,318	141,460
NORWAY
Revenue by Geographic Area [Line Items]
Revenues	51,827	39,042	44,753	90,869	74,553
Asia Pacific [Member]
Revenue by Geographic Area [Line Items]
Revenues	43,448	38,946	51,281	82,394	97,148
BRAZIL
Revenue by Geographic Area [Line Items]
Revenues	13,461	18,828	7,253	32,289	17,945
Segment, Geographical, Groups of Countries, Group Three [Member]
Revenue by Geographic Area [Line Items]
Revenues	14,811	19,639	16,744	34,450	31,348
Segment, Geographical, Groups of Countries, Foreign [Member]
Revenue by Geographic Area [Line Items]
Revenues	236,512	216,861	269,504	453,373	517,725
UNITED STATES
Revenue by Geographic Area [Line Items]
Revenues	$ 242,162	$ 199,552	$ 245,532	$ 441,714	$ 443,487