Income Taxes - Effective Tax Reconciliation (Details) - UK income tax	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Tax rates which are different than the UK and Cayman Island rates	23.40%	8.40%	14.40%
Tax impact of asset impairment	11.70%	3.90%	5.30%
Tax impact of tax restructuring	(76.10%)	0.00%	0.00%
Tax impact of tax reform	33.40%	0.00%	0.00%
Reserve for (resolution of) tax authority audits	(1.90%)	(1.00%)	1.70%
Total	(9.50%)	11.30%	21.40%