Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
OPERATING ACTIVITIES
Net income	$ 54,740	$ 47,249
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	47,609	42,681
Deferred income taxes	5,325	(9,066)
Amortization of stock-based compensation	806	2,000
Gain on disposal of property and equipment	(24,299)	(9,433)
Changes in certain working capital items:
Trade receivables	(1,557)	(6,558)
Prepaid expenses and other current assets	(2,439)	2,111
Accounts payable, accrued liabilities, and accrued expenses	(1,810)	2,257
Accrued income taxes	(5,182)	(3,736)
Net cash provided by operating activities	73,193	67,505
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	56,751	17,449
Purchases of property and equipment, net of trades	(91,845)	(25,360)
Maturity and calls of investments	9,875	31,325
Change in other assets	(195)	(4)
Net cash (used in) provided by investing activities	(25,414)	23,410
FINANCING ACTIVITIES
Payments of Dividends	(3,402)	(3,472)
Payments for Repurchase of Common Stock	0	(14,472)
Net cash used in financing activities	(3,402)	(17,944)
Net increase in cash and cash equivalents	44,377	72,971
CASH AND CASH EQUIVALENTS
Beginning of period	119,838	139,770
End of period	164,215	212,741
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds	31,076	36,246
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	19,051	1,116
Common Stock Dividends Declared in Accounts Payable	$ 1,700	$ 1,715