Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue by Geographic Area [Line Items]
Revenues	$ 519,300	$ 478,674	$ 476,120	$ 1,414,387	$ 1,437,332
Africa [Member]
Revenue by Geographic Area [Line Items]
Revenues	51,669	61,966	57,901	168,722	213,172
UNITED KINGDOM
Revenue by Geographic Area [Line Items]
Revenues	56,769	50,999	48,074	153,087	189,534
NORWAY
Revenue by Geographic Area [Line Items]
Revenues	51,952	51,827	54,412	142,820	128,965
Asia Pacific [Member]
Revenue by Geographic Area [Line Items]
Revenues	39,764	43,448	50,407	122,158	147,555
BRAZIL
Revenue by Geographic Area [Line Items]
Revenues	14,554	13,461	7,862	46,844	25,807
Segment, Geographical, Groups of Countries, Group Three [Member]
Revenue by Geographic Area [Line Items]
Revenues	45,898	14,811	23,192	80,348	54,540
Segment, Geographical, Groups of Countries, Foreign [Member]
Revenue by Geographic Area [Line Items]
Revenues	260,606	236,512	241,848	713,979	759,573
UNITED STATES
Revenue by Geographic Area [Line Items]
Revenues	$ 258,694	$ 242,162	$ 234,272	$ 700,408	$ 677,759