Employee Benefit Plan (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Value of contributed shares	$ 2,725	$ 2,708	$ 2,711
Domestic Plan [Member] | GP Retirement Savings Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Discretionary contribution shares	104,751	111,326	91,301
Value of contributed shares	$ 2,700	$ 2,700	$ 2,700
Foreign Plan [Member] | Defined Contribution Pension Schemes [Member]
Defined Contribution Plan Disclosure [Line Items]
Contributed cash	$ 2,500	$ 2,200	$ 2,400