Income Taxes - Summary Of Earliest Tax Years Open To Examination (Details)	9 Months Ended
	Sep. 30, 2018
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	2015
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	2013
Australia [Member]
Income Tax Examination [Line Items]
Earliest tax years open to examination by tax authorities	2013