Acquisitions, Divestiture, Intangible Assets and Goodwill (Acquisitions) (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended	12 Months Ended
	Oct. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Acquisitions, net of cash acquired			$ 482,853	$ 37,842	$ 393,695
Allocation of the purchase price
Goodwill		$ 1,952,444	1,952,444	1,571,335	1,745,957
Intangible assets acquired
Amortization period			34,000	38,000	37,000
Goodwill, Impairment Loss		$ 171,000	0	171,092	$ 0
Newgistics
Allocation of the purchase price
Accounts receivable	$ 36,195
Other current assets	16,051
Fixed assets	26,933
Goodwill	330,272
Intangible assets	135,640
Accounts payable and other current liabilities	(21,500)
Deferred taxes, net	(52,363)
Other assets and liabilities, net	(688)
Purchase price	470,540
Intangible assets acquired
Value	135,640
Revenue			140,000
Pro forma revenues			$ 341,000	$ 481,000
Newgistics | Customer relationships
Intangible assets acquired
Value	$ 111,600
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	10 years
Newgistics | Developed technology
Intangible assets acquired
Value	$ 19,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years
Newgistics | Tradenames
Intangible assets acquired
Value	$ 4,300
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	3 years
Newgistics | Other
Intangible assets acquired
Value	$ 740
Newgistics | Other | Minimum
Intangible assets acquired
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	1 year
Newgistics | Other | Maximum
Intangible assets acquired
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	3 years