Operations by Business Segment and Geographic Area - Revenues and Long-Lived Assets by Location (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 1,921,507	$ 2,271,603	$ 3,062,754
Long-Lived Assets	1,064,204	1,153,258
Foreign [Member]
Segment Reporting Information [Line Items]
Revenues	988,923	1,294,259	1,757,317
Long-Lived Assets	427,470	449,047
Norway [Member]
Segment Reporting Information [Line Items]
Revenues	178,712	166,180	250,272
Long-Lived Assets	78,279	83,129
Africa [Member]
Segment Reporting Information [Line Items]
Revenues	256,198	486,615	659,038
Long-Lived Assets	135,345	159,715
United Kingdom [Member]
Segment Reporting Information [Line Items]
Revenues	236,177	304,635	367,326
Long-Lived Assets	87,601	67,522
Asia Pacific [Member]
Segment Reporting Information [Line Items]
Revenues	193,865	196,679	245,978
Long-Lived Assets	50,057	61,135
Brazil [Member]
Segment Reporting Information [Line Items]
Revenues	42,607	73,280	118,056
Long-Lived Assets	50,842	55,224
Other [Member]
Segment Reporting Information [Line Items]
Revenues	81,364	66,870	116,647
Long-Lived Assets	25,346	22,322
United States [Member]
Segment Reporting Information [Line Items]
Revenues	932,584	977,344	$ 1,305,437
Long-Lived Assets	$ 636,734	$ 704,211