Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011
OPERATING ACTIVITIES
Net income	$ 47,249	$ 52,810
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	42,681	41,426
Deferred income taxes	(9,066)	13,543
Amortization of stock-based compensation	2,000	0
Gain on disposal of property and equipment	(9,433)	(22,329)
Changes in certain working capital items:
Trade receivables	(6,558)	(2,800)
Prepaid expenses and other current assets	2,111	(8,132)
Accounts payable, accrued liabilities, and accrued expenses	2,257	583
Accrued income taxes	(3,736)	(3,207)
Net cash provided by operating activities	67,505	71,894
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	17,449	49,385
Purchases of property and equipment	(25,360)	(91,274)
Maturity and calls of investments	31,325	24,225
Change in other assets	(4)	(1,726)
Net cash provided by (used in) investing activities	23,410	(19,390)
FINANCING ACTIVITIES
Cash dividends	(3,472)	(3,627)
Repurchases of common stock	(14,472)	(28,875)
Net cash used in financing activities	(17,944)	(32,502)
Net increase in cash and cash equivalents	72,971	20,002
CASH AND CASH EQUIVALENTS
Beginning of period	139,770	121,120
End of period	212,741	141,122
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds	36,246	17,165
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	1,116	5,887
Common stock dividends declared in accounts payable	1,715	1,790
Treasury stock acquired in accounts payable	$ 0	$ 768