Operating expenses (Schedule of Selling, General and Administrative Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating expenses
Wages, salaries and benefits	$ 78,796	$ 60,088
Share-based compensation expense	3,778	2,407
Buildings, facilities and technology expenses	17,343	15,591
Travel, advertising and promotion expenses	5,162	10,269
Professional fees	5,032	4,447
Other SG&A expenses	5,967	5,583
Total selling, general and administrative expenses	$ 116,078	$ 98,385