Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
OPERATING ACTIVITIES
Net income	$ 16,588	$ 14,879
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	14,105	12,531
Deferred income taxes	(3,054)	3,541
Amortization of stock-based compensation	902	0
Gain on disposal of property and equipment	(4,214)	(3,868)
Changes in certain working capital items:
Trade receivables	(5,921)	(4,991)
Prepaid expenses and other current assets	(1,445)	(7,229)
Accounts payable, accrued liabilities, and accrued expenses	2,278	5,934
Accrued income taxes	9,123	3,145
Net cash provided by operating activities	28,362	23,942
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	8,186	8,667
Purchases of property and equipment	(3,980)	(23,430)
Maturity and calls of investments	25	15,425
Change in other assets	(51)	8
Net cash (used in) provided by investing activities	4,180	670
FINANCING ACTIVITIES
Net cash used in financing activities	0	0
Net increase in cash and cash equivalents	32,542	24,612
CASH AND CASH EQUIVALENTS
Beginning of period	139,770	121,120
End of period	172,312	145,732
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds	1,264	545
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	986	3,358
Common stock dividends declared in accounts payable	$ 1,736	$ 1,814