Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
OPERATING ACTIVITIES
Net income	$ 38,874	$ 34,816
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	32,326	28,266
Deferred income taxes	4,710	(4,501)
Amortization of stock-based compensation	622	1,631
Gain on disposal of property and equipment	(19,823)	(7,760)
Changes in certain working capital items:
Trade receivables	(3,047)	(5,627)
Prepaid expenses and other current assets	(3,524)	(1,294)
Accounts payable, accrued liabilities, and accrued expenses	1,244	2,361
Accrued income taxes	(1,049)	(4,222)
Net cash provided by operating activities	50,333	43,670
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	40,404	14,656
Purchases of property and equipment, net of trades	(77,156)	(14,032)
Maturity and calls of investments	9,825	31,325
Change in other assets	11	103
Net cash (used in) provided by investing activities	(26,916)	32,052
FINANCING ACTIVITIES
Payments of Dividends	(1,701)	(1,738)
Payments for Repurchase of Common Stock	0	(3,015)
Net cash used in financing activities	(1,701)	(4,753)
Net increase in cash and cash equivalents	21,716	70,969
CASH AND CASH EQUIVALENTS
Beginning of period	119,838	139,770
End of period	141,554	210,739
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds	17,301	24,743
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	214	902
Common Stock Dividends Declared in Accounts Payable	1,701	1,735
Treasury stock acquired in accounts payable	$ 0	$ 1,969