Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating Activities:
Net income	$ 253,235	$ 138,340	$ 113,719
Noncash items included in net income:
Depreciation and amortization	141,700	134,655	119,135
Provision for doubtful accounts	3,559	4,271	2,804
Deferred income taxes	4,319	8,263	24,662
Loss (gain) from property disposals, net	(3,894)	(2,528)	(403)
Stock-based compensation	8,703	7,536	6,187
Changes in operating assets and liabilities:
Accounts receivable	(63,415)	(25,051)	(16,979)
Accounts payable	16,729	5,772	10,320
Other working capital items, net	17,757	33,344	4,203
Claims, insurance and other	13,809	1,804	7,504
Other, net	(9,910)	2,739	1,724
Net cash provided by operating activities	382,592	309,145	272,876
Investing Activities:
Acquisition of property and equipment	(285,746)	(231,142)	(287,655)
Proceeds from disposal of property and equipment	8,398	12,325	6,624
Investment in equity securities	(500)
Net cash used in investing activities	(277,848)	(218,817)	(281,031)
Financing Activities:
Repayment of revolving credit agreement	(43,175)	(369,001)	(331,188)
Borrowing of revolving credit agreement	43,175	323,072	357,117
Proceeds from stock option exercises	3,678	3,786	2,927
Shares withheld for taxes	(6,571)	(3,600)	(3,471)
Debt issuance costs			(649)
Repayment of finance leases	(20,571)	(19,525)	(18,527)
Net cash (used in) provided by financing activities	(23,464)	(65,268)	6,209
Net Increase (Decrease) in Cash and Cash Equivalents	81,280	25,060	(1,946)
Cash and cash equivalents, beginning of year	25,308	248	2,194
Cash and cash equivalents, end of year	$ 106,588	$ 25,308	248
Non Cash Investing Activities
Equipment financed with finance leases			$ 6,169