Revenue Disaggregation of Revenue - Industry (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
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	$ 2,258,349	$ 2,158,061	$ 2,089,338	$ 1,903,467	$ 1,931,146	$ 1,840,897	$ 1,788,028	$ 1,736,983	$ 8,409,215	$ 7,297,054	$ 6,758,138
Supply Chain Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									2,398,144	1,937,352
Automotive | Supply Chain Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									947,408	783,640	724,082
Technology and healthcare | Supply Chain Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									480,026	387,252	283,836
CPG and retail | Supply Chain Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									766,765	535,689	452,096
Industrial and other | Supply Chain Solutions
Disaggregation of Revenue [Line Items]
Revenue from contract with customer									$ 203,945	$ 230,771	$ 149,342