Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 55,439	$ 16,835	$ 42,085
Adjustments to reconcile net income to net cash provided by operating activities:
Provision (reversal) for losses on accounts receivable	454	(241)	1,100
Reversal of gain on sales to equity method investee	(179)	(207)	(26)
Depreciation and amortization	72,422	71,647	62,010
Amortization of deferred financing fees	242	293	261
Unrealized (gain) loss on ineffective portion of fuel hedges			(1,454)
Return of (issuance of) cash collateral on fuel hedge			5,000
Deferred income tax (benefit) expense	(23,023)	(922)	20,701
Income tax benefit arising from restricted share vesting and stock options exercised	457	1,108
Casualty premium credit			(3,600)
Income from equity method investment	(3,400)	(3,000)	(4,570)
Return on investment in affiliated company	1,960	1,470
Loss (gain) on disposition of property and equipment	4,024	808	(626)
Stock-based compensation expense	1,201	1,378	1,496
Changes in operating assets and liabilities:
Receivables and advances	(23,670)	21,207	(28,120)
Prepaid expenses and other assets	1,768	(1,464)	2,688
Inventory and supplies	(252)	24	398
Insurance and claims accrual	(1,165)	(1,390)	(1,304)
Accounts payable and accrued expenses	(3,425)	(5,116)	(10,562)
Net cash flows provided by operating activities	82,853	102,430	85,477
Cash flows from investing activities:
Acquisition of property and equipment	(110,802)	(112,794)	(181,963)
Proceeds from disposition of property and equipment	48,749	65,507	34,287
Net cash flows used by investing activities	(62,053)	(47,287)	(147,676)
Cash flows from financing activities:
Change in checks outstanding in excess of bank balances	(189)	(4,509)	4,698
Proceeds from issuance of notes payable	121,210	69,432	113,077
Proceeds from exercise of stock options		32	1,092
Repayments of notes payable	(122,676)	(120,630)	(67,276)
Repayments of capital lease obligations	(7,416)	(4,140)	(1,718)
Proceeds under revolving credit facility	1,271,669	1,023,978	870,432
Repayments under revolving credit facility	(1,274,847)	(1,014,796)	(867,430)
Common stock repurchased			(4,994)
Payment of minimum tax withholdings on stock compensation	(785)	(1,142)	(2,280)
Debt refinancing costs	(160)	(108)	(242)
Net cash flows (used in) provided by financing activities	(13,194)	(51,883)	45,359
Net change in cash and cash equivalents	7,606	3,260	(16,840)
Cash and cash equivalents at beginning of year	7,750	4,490	21,330
Cash and cash equivalents at end of year	15,356	7,750	4,490
Supplemental disclosure of cash flow information:
Interest, net of capitalized interest	8,268	8,453	8,371
Income taxes	(2,222)	6,412	8,112
Equipment purchased under capital leases	$ 9,953	$ 11,765	$ 1,318