Revenue (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue from Contracts with Customer [Line Items]
Cumulative effect on retained earnings	$ 2,000
Accounts receivable, net	301,967			$ 304,174
Contract assets	8,100		$ 7,800
Contract liabilities	3,300			$ 2,100
Revenue recognized from contract liability during the period	$ 2,100
Average transit time	3 days
Revenues	$ 562,684	$ 501,221
Operating expenses	527,569	475,249
Difference between revenue guidance in effect before and after Topic 606
Revenue from Contracts with Customer [Line Items]
Revenues	300
Operating expenses	100
Other revenues
Revenue from Contracts with Customer [Line Items]
Revenues	14,166	$ 16,532
Other revenues | Difference between revenue guidance in effect before and after Topic 606
Revenue from Contracts with Customer [Line Items]
Revenues	$ (3,100)