Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net earnings	$ 506,035	$ 516,320	$ 489,585
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	527,375	499,145	435,893
Noncash lease expense	45,985	39,517	0
Share-based compensation	60,698	53,324	47,369
Loss on sale of revenue equipment and other	4,389	13,057	12,107
Deferred income taxes	(7,056)	55,617	101,591
Changes in operating assets and liabilities:
Trade accounts receivable	(109,758)	50,310	(130,931)
Income taxes receivable or payable	57,851	41,447	(41,071)
Other current assets	(18,038)	(4,975)	(6,133)
Trade accounts payable	(5,482)	(85,327)	98,037
Claims accruals	(9,072)	(20,727)	21,580
Accrued payroll and other accrued expenses	69,932	(59,361)	59,814
Net cash provided by operating activities	1,122,859	1,098,347	1,087,841
Cash flows from investing activities:
Additions to property and equipment	(738,545)	(854,115)	(995,650)
Proceeds from sale of equipment	137,776	165,918	110,165
Business acquisition	(12,136)	(115,654)	0
Change in other assets	(52)	(111)	(1,288)
Net cash used in investing activities	(612,957)	(803,962)	(886,773)
Cash flows from financing activities:
Proceeds from long-term debt	0	700,000	0
Payments on long-term debt	0	(250,000)	0
Proceeds from revolving lines of credit and other	222,124	1,591,014	3,204,715
Payments on revolving lines of credit and other	(220,100)	(1,904,000)	(3,137,900)
Purchase of treasury stock	(92,548)	(275,657)	(150,338)
Stock repurchased for payroll taxes and other	(26,842)	(16,525)	(19,563)
Dividends paid	(114,234)	(111,817)	(104,994)
Net cash used in financing activities	(231,600)	(266,985)	(208,080)
Net increase/(decrease) in cash and cash equivalents	278,302	27,400	(7,012)
Cash and cash equivalents at beginning of year	35,000	7,600	14,612
Cash and cash equivalents at end of year	313,302	35,000	7,600
Supplemental disclosure of cash flow information:
Interest	48,351	46,721	39,901
Income taxes	95,454	71,681	83,822
Noncash investing activities
Accruals for equipment received	$ 12,533	$ 25,505	$ 49,390