Revenue (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue from Contracts with Customer [Line Items]
Cumulative effect on retained earnings			$ 2,000
Accounts receivable, net	$ 333,134		333,134			$ 304,174
Contract assets	8,800		8,800		$ 7,800
Contract liabilities	$ 2,400		2,400			$ 2,100
Revenue recognized from contract liability during the period			2,100
Average transit time	3 days
Revenues	$ 619,130	$ 519,508	1,181,814	$ 1,020,729
Operating expenses	568,347	482,595	1,095,916	957,844
Difference between revenue guidance in effect before and after Topic 606
Revenue from Contracts with Customer [Line Items]
Revenues	700		1,000
Operating expenses	300		400
Other revenues
Revenue from Contracts with Customer [Line Items]
Revenues	15,053	$ 15,651	29,219	$ 32,183
Other revenues | Difference between revenue guidance in effect before and after Topic 606
Revenue from Contracts with Customer [Line Items]
Revenues	$ (3,900)		$ (7,100)