Business Segments Segment Reporting Information (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]
Revenues	$ 929,650	$ 906,449	$ 878,509	$ 685,173	$ 679,035	$ 667,048	$ 653,313	$ 626,528	$ 3,399,781	$ 2,625,924	$ 2,440,413
Gross profit	$ 271,896	$ 270,172	$ 263,846	$ 217,737	$ 220,677	$ 218,315	$ 212,937	$ 198,144	1,023,651	850,073	795,183
Operating Income (Loss)									260,148	224,736	189,726
Amortization of intangible assets									58,506	33,444	39,628
Apex
Segment Reporting Information [Line Items]
Revenues									2,300,242	2,037,154	1,836,488
Gross profit									687,861	606,294	548,421
Operating Income (Loss)									262,297	222,010	195,133
Amortization of intangible assets									26,185	29,361	34,359
Oxford
Segment Reporting Information [Line Items]
Revenues									606,510	588,770	603,925
Gross profit									248,875	243,779	246,762
Operating Income (Loss)									54,142	53,787	51,294
Amortization of intangible assets									4,179	4,083	5,269
ECS
Segment Reporting Information [Line Items]
Revenues									493,029	0	0
Gross profit									86,915
Operating Income (Loss)									15,514
Amortization of intangible assets									28,142
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual									493,029
Corporate
Segment Reporting Information [Line Items]
Operating Income (Loss)									$ (71,805)	$ (51,061)	$ (56,701)