Accounts Receivable Accounts Receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	$ 430,210	$ 375,993
Allowance for doubtful accounts	(13,505)	(12,070)	$ (6,213)	$ (5,826)
Accounts receivable, net	416,705	363,923
Accounts receivable - trade [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	401,723	353,347
Accounts receivable - grants from outside parties [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	17,734	10,652
Accounts receivable - insurance and other third party claims [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total accounts receivable	10,753	$ 11,994
Pentalver Transport Limited [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounts receivable, net	$ 22,600