Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011
OPERATING ACTIVITIES
Net income	$ 34,816	$ 37,412
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	28,266	26,360
Deferred income taxes	(4,501)	9,533
Amortization of stock-based compensation	1,631	0
Gain on disposal of property and equipment	(7,760)	(15,530)
Changes in certain working capital items:
Trade receivables	(5,627)	(5,268)
Prepaid expenses and other current assets	(1,294)	(9,508)
Accounts payable, accrued liabilities, and accrued expenses	2,361	1,850
Accrued income taxes	(4,222)	(3,070)
Net cash provided by operating activities	43,670	41,779
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	14,656	30,893
Purchases of property and equipment	(14,032)	(60,742)
Maturity and calls of investments	31,325	21,650
Change in other assets	103	(1,469)
Net cash provided by (used in) investing activities	32,052	(9,668)
FINANCING ACTIVITIES
Cash dividends	(1,738)	(1,814)
Repurchases of common stock	(3,015)	0
Net cash used in financing activities	(4,753)	(1,814)
Net increase in cash and cash equivalents	70,969	30,297
CASH AND CASH EQUIVALENTS
Beginning of period	139,770	121,120
End of period	210,739	151,417
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds	24,743	11,131
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	902	4,738
Common stock dividends declared in accounts payable	1,735	1,814
Treasury stock acquired in accounts payable	$ 1,969	$ 0