Revenue - Revenue by Geographic Area (Details) - USD ($) $ in Thousands	3 Months Ended			12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2018
Revenue by Geographic Area [Line Items]
Revenues	$ 493,886	$ 495,095	$ 416,413	$ 495,095
Africa [Member]
Revenue by Geographic Area [Line Items]
Revenues	87,106	71,237	55,087
United Kingdom [Member]
Revenue by Geographic Area [Line Items]
Revenues	53,298	50,304	45,319
Norway [Member]
Revenue by Geographic Area [Line Items]
Revenues	42,466	42,731	39,042
Asia Pacific [Member]
Revenue by Geographic Area [Line Items]
Revenues	41,426	41,685	38,946
Brazil [Member]
Revenue by Geographic Area [Line Items]
Revenues	17,763	17,161	18,828
Other Geographical [Member]
Revenue by Geographic Area [Line Items]
Revenues	21,222	23,200	19,639
Non-US [Member]
Revenue by Geographic Area [Line Items]
Revenues	263,281	246,318	216,861
UNITED STATES
Revenue by Geographic Area [Line Items]
Revenues	$ 230,605	$ 248,777	$ 199,552