Condensed Consolidated Statements of Cash Flows (Unaudited) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013
Cash flows from operating activities:
Net earnings	$ 162,071	$ 161,045
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	140,693	123,013
Share-based compensation	18,814	17,977
Gain on sale of revenue equipment and other	(5,110)	(1,442)
Benefit from deferred income taxes	(17,672)	(3,473)
Changes in operating assets and liabilities:
Trade accounts receivable	(99,101)	(113,632)
Other assets	25,441	11,006
Trade accounts payable	46,858	97,473
Income taxes payable or receivable	7,500	2,120
Claims accruals	13,000	9,167
Accrued payroll and other accrued expenses	8,507	(9,683)
Net cash provided by operating activities	301,001	293,571
Cash flows from investing activities:
Additions to property and equipment	(438,013)	(238,494)
Net proceeds from sale of equipment	85,161	26,802
Changes in other assets	9	(1)
Net cash used in investing activities	(352,843)	(211,693)
Cash flows from financing activities:
Proceeds from long-term debt	499,642
Payments on long-term debt	(250,000)	(50,000)
Proceeds from revolving lines of credit and other	869,221	846,985
Payments on revolving lines of credit and other	(954,763)	(809,885)
Purchase of treasury stock	(75,000)	(64,723)
Stock option exercises and other	4,611	6,208
Stock repurchased for payroll taxes	(520)	(573)
Tax benefit of stock options exercised	5,437	7,999
Dividends paid	(46,747)	(17,555)
Net cash provided by/(used in) financing activities	51,881	(81,544)
Net change in cash and cash equivalents	39	334
Cash and cash equivalents at beginning of period	5,831	5,589
Cash and cash equivalents at end of period	5,870	5,923
Cash paid during the period for:
Interest	12,306	12,695
Income taxes	$ 93,853	$ 91,074