Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets, Current [Abstract]
Cash and cash equivalents	$ 11,472	$ 34,787
Customer receivables, less allowances of $8,976 and $9,069, respectively	310,501	303,170
Other receivables	34,547	44,730
Prepaid expenses and other current assets	25,210	21,085
Total current assets	381,730	403,772
Property and equipment
Revenue equipment	1,358,317	1,158,108
Land and structures	1,221,250	1,088,372
Other fixed assets	365,673	321,310
Leasehold improvements	7,585	6,982
Total property and equipment	2,952,825	2,574,772
Less: Accumulated depreciation	(929,377)	(831,527)
Net property and equipment	2,023,448	1,743,245
Goodwill	19,463	19,463
Other assets	41,863	40,386
Total assets	2,466,504	2,206,866
Liabilities, Current [Abstract]
Accounts Payable, Current	66,774	45,314
Compensation and benefits	124,589	106,200
Claims and insurance accruals	44,917	42,271
Other accrued liabilities	22,634	26,139
Current maturities of long-term debt	26,488	35,714
Total current liabilities	285,402	255,638
Long-term debt	107,317	120,000
Other non-current liabilities	154,094	145,752
Deferred income taxes	235,054	191,412
Total long-term liabilities	496,465	457,164
Total liabilities	781,867	712,802
Shareholders equity
Common stock - $0.10 par value, 140,000,000 shares authorized, 84,411,878 and 86,094,297 shares outstanding at December 31, 2015 and 2014, respectively	8,441	8,609
Capital in excess of par value	134,401	134,401
Retained earnings	1,541,795	1,351,054
Total shareholders' equity	1,684,637	1,494,064
Total liabilities and shareholders' equity	$ 2,466,504	$ 2,206,866