REVENUE Disaggregation of Revenue - Industry (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Total revenue	$ 2,180,327	$ 1,904,205
SCS
Disaggregation of Revenue [Line Items]
Total revenue	635,671	494,707
Automotive | SCS
Disaggregation of Revenue [Line Items]
Total revenue	253,679	207,792
Technology and healthcare | SCS
Disaggregation of Revenue [Line Items]
Total revenue	113,668	103,097
CPG and retail | SCS
Disaggregation of Revenue [Line Items]
Total revenue	217,098	135,358
Industrial and other | SCS
Disaggregation of Revenue [Line Items]
Total revenue	$ 51,226	$ 48,460