Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 76,025	$ 27,289
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	65,990	60,774
Stock-based compensation	4,052	4,213
Deferred income taxes	(24,148)	13,572
Net gain on sale and impairment of assets	(1,036)	(427)
Changes in assets and liabilities which provided/(used) cash, net of effect of acquisitions:
Accounts receivable, net	(6,299)	6,524
Materials and supplies	2,593	(2,140)
Prepaid expenses and other	(7,025)	(2,226)
Accounts payable and accrued expenses	(12,381)	(29,330)
Other assets and liabilities, net	3,588	6,767
Net cash provided by operating activities	101,359	85,016
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(58,222)	(34,738)
Grant proceeds from outside parties	5,934	4,771
Net cash provided by acquisitions	0	2,935
Insurance proceeds for the replacement of assets	1,600	1,406
Proceeds from disposition of property and equipment	1,423	928
Net cash used in investing activities	(49,265)	(24,698)
CASH FLOWS FROM FINANCING ACTIVITIES:
Principal payments on revolving line-of-credit, long-term debt and capital lease obligations	(121,850)	(167,730)
Proceeds from revolving line-of-credit and long-term borrowings	176,840	112,294
Common share repurchases	(57,376)	0
Installment payments on Freightliner deferred consideration	(6,255)	0
Other financing related activities, net	(1,973)	(814)
Net cash used in financing activities	(10,614)	(56,250)
EFFECT OF EXCHANGE RATE CHANGES ON CASH AND CASH EQUIVALENTS	(562)	1,878
INCREASE IN CASH AND CASH EQUIVALENTS	40,918	5,946
CASH AND CASH EQUIVALENTS, beginning of period	80,472	32,319
CASH AND CASH EQUIVALENTS, end of period	$ 121,390	$ 38,265