Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
OPERATING ACTIVITIES
Net income	$ 69,932	$ 62,216	$ 56,949
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	57,876	61,949	58,730
Deferred income taxes	14,743	(8,440)	14,637
Amortization of stock-based compensation	150	0	0
Gain on disposal of property and equipment	(32,133)	(13,317)	(19,708)
Changes in certain working capital items:
Trade receivables	(2,579)	(4,258)	(558)
Prepaid expenses and other current assets	(6,459)	252	671
Accounts payable, accrued liabilities, and accrued expenses	(952)	1,609	(567)
Accrued income taxes	(1,498)	(1,404)	(9,051)
Net cash provided by operating activities	99,080	98,607	101,103
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	73,018	21,649	11
Purchases of property and equipment, net of trades	(126,257)	(14,551)	(79,123)
Maturity and calls of investments	38,125	79,225	27,000
Purchases of investments	0	(18,000)	(350)
Change in other assets	(1,818)	(217)	(311)
Net cash (used in) provided by investing activities	(16,932)	68,106	(52,773)
FINANCING ACTIVITIES
Cash dividends	(7,148)	(97,944)	(7,270)
Repurchases of common stock	(56,350)	0	(45,360)
Net cash used in financing activities	(63,498)	(97,944)	(52,630)
Net increase in cash and cash equivalents	18,650	68,769	(4,300)
CASH AND CASH EQUIVALENTS
Beginning of period	121,120	52,351	56,651
End of period	139,770	121,120	52,351
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds	24,152	40,502	18,767
Noncash investing and financing activities:
Fair value of revenue equipment traded	0	14,604	60,645
Purchased property and equipment in accounts payable	$ 1,683	$ 1,190	$ 178