Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net earnings	$ 516,320	$ 489,585	$ 686,263
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	499,145	435,893	383,518
Noncash lease expense	39,517	0	0
Share-based compensation	53,324	47,369	38,291
Loss on sale of revenue equipment and other	13,057	12,107	7,370
Advance deposit impairment	0	0	20,240
Deferred income taxes	55,617	101,591	(248,764)
Changes in operating assets and liabilities:
Trade accounts receivable	50,310	(130,931)	(166,111)
Income taxes receivable or payable	41,447	(41,071)	(45,542)
Other current assets	(4,975)	(6,133)	69,462
Trade accounts payable	(85,327)	98,037	85,237
Claims accruals	(20,727)	21,580	25,021
Accrued payroll and other accrued expenses	(59,361)	59,814	168
Net cash provided by operating activities	1,098,347	1,087,841	855,153
Cash flows from investing activities:
Additions to property and equipment	(854,115)	(995,650)	(526,928)
Proceeds from sale of equipment	165,918	110,165	16,413
Business acquisition	(115,654)	0	(136,879)
Change in other assets	(111)	(1,288)	(3,888)
Net cash used in investing activities	(803,962)	(886,773)	(651,282)
Cash flows from financing activities:
Proceeds from long-term debt	700,000	0	0
Payments on long-term debt	(250,000)	0	0
Proceeds from revolving lines of credit and other	1,591,014	3,204,715	2,716,155
Payments on revolving lines of credit and other	(1,904,000)	(3,137,900)	(2,612,501)
Purchase of treasury stock	(275,657)	(150,338)	(179,813)
Stock repurchased for payroll taxes and other	(16,525)	(19,563)	(18,115)
Dividends paid	(111,817)	(104,994)	(101,362)
Net cash used in financing activities	(266,985)	(208,080)	(195,636)
Net increase/(decrease) in cash and cash equivalents	27,400	(7,012)	8,235
Cash and cash equivalents at beginning of year	7,600	14,612	6,377
Cash and cash equivalents at end of year	35,000	7,600	14,612
Supplemental disclosure of cash flow information:
Interest	46,721	39,901	28,785
Income taxes	71,681	83,822	190,783
Noncash investing activities
Accruals for equipment received	$ 25,505	$ 49,390	$ 53,026