Consolidated Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net earnings	$ 427,235,000	$ 374,792,000	$ 342,382,000
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	339,613,000	294,496,000	253,380,000
Share-based compensation	37,228,000	35,333,000	32,354,000
Gain on sale of revenue equipment and other	(1,281,000)	(6,342,000)	(5,334,000)
Provision for deferred income taxes	80,427,000	79,343,000	48,076,000
Changes in operating assets and liabilities:
Trade accounts receivable	(747,000)	(85,276,000)	(102,508,000)
Income taxes receivable or payable	3,055,000	(72,291,000)	(5,381,000)
Other current assets	(17,735,000)	(29,793,000)	(23,254,000)
Trade accounts payable	8,600,000	15,284,000	11,530,000
Claims accruals	7,502,000	28,498,000	20,779,000
Accrued payroll and other accrued expenses	(10,589,000)	12,735,000	2,327,000
Net cash provided by operating activities	873,308,000	646,779,000	574,351,000
Cash flows from investing activities:
Additions to property and equipment	(725,122,000)	(808,569,000)	(493,431,000)
Proceeds from sale of equipment	168,686,000	148,859,000	50,927,000
Change in other assets	(20,096,000)	29,000	(37,000)
Net cash used in investing activities	(576,532,000)	(659,681,000)	$ (442,541,000)
Cash flows from financing activities:
Proceeds from issuances of long-term debt	349,129,000	499,642,000
Payments on long-term debt	(250,000,000)	(250,000,000)	$ (100,000,000)
Proceeds from revolving lines of credit and other	2,110,800,000	2,092,193,000	1,933,753,000
Payments on revolving lines of credit and other	(2,138,466,000)	(2,110,749,000)	(1,811,177,000)
Purchase of treasury stock	(262,275,000)	(125,000,000)	(114,723,000)
Stock option exercises and other	2,978,000	7,324,000	9,403,000
Stock repurchased for payroll taxes	(24,850,000)	(23,419,000)	(17,963,000)
Tax benefit of stock options exercised	12,877,000	16,645,000	21,950,000
Dividends paid	(97,364,000)	(93,604,000)	(52,811,000)
Net cash provided by/(used in) financing activities	(297,171,000)	13,032,000	(131,568,000)
Net increase/(decrease) in cash and cash equivalents	(395,000)	130,000	242,000
Cash and cash equivalents at beginning of year	5,961,000	5,831,000	5,589,000
Cash and cash equivalents at end of year	5,566,000	5,961,000	5,831,000
Cash paid during the year for:
Interest	27,245,000	26,685,000	24,722,000
Income taxes	$ 163,304,000	$ 192,955,000	$ 141,968,000