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Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Reportable Segments - Operations of Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue	$ 357,984	$ 355,452	$ 358,624	$ 346,293	$ 342,586	$ 341,053	$ 340,309	$ 333,992	$ 1,418,353	$ 1,357,940	$ 1,319,706
Gross profit	$ 106,556	$ 104,381	$ 107,483	$ 100,188	$ 102,627	$ 104,375	$ 103,919	$ 97,135	418,608	408,056	408,499
Operating and other expenses									341,455	343,962	352,544
Income before income taxes									77,153	64,094	55,955
Tech
Segment Reporting Information [Line Items]
Revenue									990,089	907,511	883,477
Gross profit									277,388	257,118	255,842
FA
Segment Reporting Information [Line Items]
Revenue									313,848	346,135	337,601
Gross profit									109,099	118,479	120,551
GS
Segment Reporting Information [Line Items]
Revenue									114,416	104,294	98,628
Gross profit									$ 32,121	$ 32,459	$ 32,106