Operating expenses (Schedule of Depreciation and Amortization Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Operating expenses
Depreciation expense	$ 7,536	$ 7,157	$ 15,573	$ 14,325
Amortization expense	10,321	9,955	21,577	19,902
Total depreciation and amortization expenses	$ 17,857	$ 17,112	$ 37,150	$ 34,227