Operating expenses (Schedule of Selling, General and Administrative Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating expenses
Employee compensation expenses	$ 62,495	$ 61,464
Buildings, facilities and technology expenses	15,591	15,915
Travel, advertising and promotion expenses	10,269	9,142
Professional fees	4,447	4,075
Other SG&A expenses	5,583	4,588
Total selling, general and administrative expenses	$ 98,385	$ 95,184