Operating expenses (Schedule of Depreciation and Amortization Expenses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating expenses
Depreciation expense	$ 31,330	$ 29,112	$ 29,021
Amortization expense	43,591	41,389	37,593
Total depreciation and amortization expenses	$ 74,921	$ 70,501	$ 66,614