Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Assets, Current [Abstract]
Cash and cash equivalents	$ 30,174	$ 12,857
Customer receivables, less allowances of $8,067 and $8,561, respectively	248,069	219,039
Other receivables	10,225	1,324
Prepaid expenses and other current assets	21,262	21,754
Deferred income taxes	23,249	20,054
Total current assets	332,979	275,028
Property and equipment
Revenue equipment	1,009,936	922,030
Land and structures	990,256	874,768
Other fixed assets	266,563	225,298
Leasehold improvements	6,378	6,128
Total property and equipment	2,273,133	2,028,224
Less: Accumulated depreciation	(730,074)	(648,919)
Net property and equipment	1,543,059	1,379,305
Goodwill	19,463	19,463
Other assets	36,588	38,718
Total assets	1,932,089	1,712,514
Liabilities, Current [Abstract]
Accounts Payable, Current	36,788	44,891
Compensation and benefits	97,187	80,047
Claims and insurance accruals	38,784	33,990
Other accrued liabilities	21,480	20,906
Income taxes payable	2,168	6,327
Current maturities of long-term debt	35,715	38,978
Total current liabilities	232,122	225,139
Long-term debt	155,714	201,429
Other non-current liabilities	123,054	106,791
Deferred income taxes	189,117	153,186
Total long-term liabilities	467,885	461,406
Total liabilities	700,007	686,545
Commitments and contingent liabilities
Shareholders equity
Common stock - $0.10 par value, 140,000,000 shares authorized, 86,164,917 shares outstanding at December 31, 2013 and 2012	8,616	8,616
Capital in excess of par value	134,401	134,401
Retained earnings	1,089,065	882,952
Total shareholders' equity	1,232,082	1,025,969
Total liabilities and shareholders' equity	$ 1,932,089	$ 1,712,514