Income Taxes - Summary Of Earliest Tax Years Open To Examination (Details)	12 Months Ended
	Dec. 31, 2017
United States [Member]
Income Tax Examination [Line Items]
Earliest tax years open to examination by tax authorities	2014
United Kingdom [Member]
Income Tax Examination [Line Items]
Earliest tax years open to examination by tax authorities	2015
Norway [Member]
Income Tax Examination [Line Items]
Earliest tax years open to examination by tax authorities	2007
Angola Member
Income Tax Examination [Line Items]
Earliest tax years open to examination by tax authorities	2013
Brazil [Member]
Income Tax Examination [Line Items]
Earliest tax years open to examination by tax authorities	2012
Australia [Member]
Income Tax Examination [Line Items]
Earliest tax years open to examination by tax authorities	2013