Pensions (Details) £ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2016 GBP (£)	May 31, 2014 GBP (£)
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plans, Estimated Future Employer Contributions in Next Fiscal Year	$ 1,565	£ 1,272
Estimated net loss amortized from accumulated other comprehensive income (loss) into net periodic pension cost in the next fiscal year	744
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Payment Obligation Guaranteed by Parent Company	$ 10,086		£ 8,200