Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
OPERATING ACTIVITIES
Net income	$ 19,734	$ 16,588
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	15,232	14,105
Deferred income taxes	3,178	(3,054)
Amortization of stock-based compensation	354	902
Gain on disposal of property and equipment	(11,178)	(4,214)
Changes in certain working capital items:
Trade receivables	(5,192)	(5,921)
Prepaid expenses and other current assets	(2,052)	(1,445)
Accounts payable, accrued liabilities, and accrued expenses	1,393	2,278
Accrued income taxes	6,985	9,123
Net cash provided by operating activities	28,454	28,362
INVESTING ACTIVITIES
Proceeds from sale of property and equipment	17,452	8,186
Purchases of property and equipment, net of trades	(38,232)	(3,980)
Maturity and calls of investments	25	25
Change in other assets	(31)	(51)
Net cash (used in) provided by investing activities	(20,786)	4,180
FINANCING ACTIVITIES
Net cash used in financing activities	0	0
Net increase in cash and cash equivalents	7,668	32,542
CASH AND CASH EQUIVALENTS
Beginning of period	119,838	139,770
End of period	127,506	172,312
SUPPLEMENTAL DISCLOSURES OF CASH FLOW INFORMATION
Cash paid during the period for income taxes, net of refunds	434	1,264
Noncash investing and financing activities:
Purchased property and equipment in accounts payable	18,752	986
Common Stock Dividends Declared in Accounts Payable	$ 1,701	$ 1,736