Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets, Current [Abstract]
Cash and cash equivalents	$ 34,787	$ 30,174
Customer receivables, less allowances of $9,069 and $8,067, respectively	303,170	248,069
Other receivables	44,730	10,225
Prepaid expenses and other current assets	21,085	21,262
Deferred income taxes	29,371	23,249
Total current assets	433,143	332,979
Property and equipment
Revenue equipment	1,158,108	1,009,936
Land and structures	1,088,372	990,256
Other fixed assets	321,310	266,563
Leasehold improvements	6,982	6,378
Total property and equipment	2,574,772	2,273,133
Less: Accumulated depreciation	(831,527)	(730,074)
Net property and equipment	1,743,245	1,543,059
Goodwill	19,463	19,463
Other assets	40,386	36,588
Total assets	2,236,237	1,932,089
Liabilities, Current [Abstract]
Accounts Payable, Current	45,314	36,788
Compensation and benefits	106,200	97,187
Claims and insurance accruals	42,271	38,784
Other accrued liabilities	26,139	21,480
Income taxes payable	0	2,168
Current maturities of long-term debt	35,714	35,715
Total current liabilities	255,638	232,122
Long-term debt	120,000	155,714
Other non-current liabilities	145,752	123,054
Deferred income taxes	220,783	189,117
Total long-term liabilities	486,535	467,885
Total liabilities	742,173	700,007
Shareholders equity
Common stock - $0.10 par value, 140,000,000 shares authorized, 86,094,297 and 86,164,917 shares outstanding at December 31, 2014 and 2013, respectively	8,609	8,616
Capital in excess of par value	134,401	134,401
Retained earnings	1,351,054	1,089,065
Total shareholders' equity	1,494,064	1,232,082
Total liabilities and shareholders' equity	$ 2,236,237	$ 1,932,089