Operating Expenses (Schedule of Selling, General and Administrative Expenses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Expenses [Abstract]
Employee compensation expenses	$ 208,370	$ 180,929	$ 166,227
Buildings, facilities and technology expenses	53,151	49,219	41,404
Travel, advertising and promotion expense	30,440	24,384	22,307
Professional fees	13,522	13,344	12,500
Other SG&A expenses	17,787	15,653	11,951
Total selling, general and administrative expenses	$ 323,270	$ 283,529	$ 254,389