Operations by Business Segment and Geographic Area - Revenues and Long-Lived Assets by Location (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues	$ 1,909,482	$ 1,921,507	$ 2,271,603
Foreign [Member]
Segment Reporting Information [Line Items]
Revenues	960,297	988,923	1,294,259
Norway [Member]
Segment Reporting Information [Line Items]
Revenues	185,552	178,712	166,180
Africa [Member]
Segment Reporting Information [Line Items]
Revenues	239,959	256,198	486,615
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	203,391	236,177	304,635
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Revenues	163,843	193,865	196,679
Brazil [Member]
Segment Reporting Information [Line Items]
Revenues	64,004	42,607	73,280
Other [Member]
Segment Reporting Information [Line Items]
Revenues	103,548	81,364	66,870
United States [Member]
Segment Reporting Information [Line Items]
Revenues	$ 949,185	$ 932,584	$ 977,344