Operating expenses (Schedule of Selling, General and Administrative Expenses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating expenses
Employee compensation expenses	$ 286,908	$ 246,028	$ 249,115
Buildings, facilities and technology expenses	62,755	61,177	60,930
Travel, advertising and promotion expenses	25,780	38,248	36,728
Professional fees	18,220	15,572	16,768
Other SG&A expenses	23,860	21,364	19,135
Total selling, general and administrative expenses	$ 417,523	$ 382,389	$ 382,676