Employee Benefit Plan (Details Textual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Contribution Plan Disclosure [Line Items]
Value of contributed shares	$ 2,708	$ 2,711	$ 2,469
United States Postretirement Benefit Plan of US Entity [Member] | GP Retirement Savings Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Discretionary contribution shares	111,326	91,301	90,876
Value of contributed shares	$ 2,700	$ 2,700	$ 2,500
Foreign Postretirement Benefit Plan [Member] | Defined Contribution Pension Schemes [Member]
Defined Contribution Plan Disclosure [Line Items]
Contributed cash	$ 2,200	$ 2,400	$ 1,900