Operating Expenses (Schedule of Selling, General and Administrative Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating Expenses [Abstract]
Employee compensation expenses	$ 56,959	$ 55,560	$ 186,951	$ 147,420
Buildings, facilities and technology expenses	15,058	13,494	45,767	39,088
Travel, advertising and promotion expenses	9,302	8,431	27,821	21,212
Professional fees	2,983	3,381	12,638	9,705
Other SG&A expenses	4,021	4,469	13,875	12,862
Total selling, general and administrative expenses	$ 88,323	$ 85,335	$ 287,052	$ 230,287