CONDENSED CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Current Assets:
Cash	$ 12,190	$ 2,849
Short-term investments	76,214	50,601
Accounts receivable - net
Trade	155,709	137,179
Other	1,869	3,974
Inventories of spare parts-net	67,403	66,074
Prepaid expenses	11,493	10,137
Work in progress	77,921	77,764
Deferred income taxes	11,967	11,967
Income taxes receivable	1,623	1,613
Other current assets		988
Total current assets	416,389	363,146
Property and equipment-net	756,928	749,501
Restricted investments	14,685	14,685
Other	15,853	20,562
Total assets	1,203,855	1,147,894
Current Liabilities:
Accounts payable	40,754	26,308
Accrued and other current liabilities	122,111	117,162
Total current liabilities	162,865	143,470
Long-term debt	375,723	386,755
Deferred income taxes	124,303	105,418
Other long-term liabilities	13,061	12,636
Commitments and contingencies (Note 9)
Shareholders' Equity:
Additional paid-in capital	295,528	295,582
Accumulated other comprehensive loss	(80)	(51)
Retained earnings	230,907	202,553
Total shareholders' equity	527,903	499,615
Total liabilities and shareholders' equity	1,203,855	1,147,894
Voting Common Stock [Member]
Shareholders' Equity:
Common stock	291	285
Total shareholders' equity	291	285
Non-Voting Common Stock [Member]
Shareholders' Equity:
Common stock	1,257	1,246
Total shareholders' equity	$ 1,257	$ 1,246