Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
OPERATING ACTIVITIES
Net income	$ 61,541	$ 69,932	$ 62,216
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	57,821	57,876	61,949
Deferred income taxes	(5,751)	14,743	(8,440)
Amortization of stock-based compensation	2,379	150	0
Gain on disposal of property and equipment	(15,109)	(32,133)	(13,317)
Changes in certain working capital items:
Trade receivables	(2,357)	(2,579)	(4,258)
Prepaid expenses and other current assets	5,688	(6,459)	252
Accounts payable, accrued liabilities, and accrued expenses	953	(952)	1,609
Accrued income taxes	(2,992)	(1,498)	(1,404)
Net cash provided by operating activities	102,173	99,080	98,607
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	29,184	73,018	21,649
Purchases of property and equipment, net of trades	(66,811)	(126,257)	(14,551)
Maturity and calls of investments	32,350	38,125	79,225
Purchases of investments	0	0	(18,000)
Change in other assets	(704)	(1,818)	(217)
Net cash (used in) provided by investing activities	(5,981)	(16,932)	68,106
FINANCING ACTIVITIES
Cash dividends	(91,934)	(7,148)	(97,944)
Repurchases of common stock	(24,190)	(56,350)	0
Net cash used in financing activities	(116,124)	(63,498)	(97,944)
Net (decrease) increase in cash and cash equivalents	(19,932)	18,650	68,769
CASH AND CASH EQUIVALENTS
Beginning of period	139,770	121,120	52,351
End of period	119,838	139,770	121,120
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds	42,776	24,152	40,502
Noncash investing and financing activities:
Fair value of revenue equipment traded	0	0	14,604
Purchased property and equipment in accounts payable	$ 698	$ 1,683	$ 1,190