Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Current Assets:
Cash	$ 6,270	$ 934
Short-term investments	185,244	85,916
Accounts receivable - net
Trade	178,833	172,538
Other	1,928	1,182
Inventories of spare parts - net	73,793	71,137
Prepaid expenses	9,314	8,357
Deferred income taxes	9,915	12,723
Income taxes receivable	1,227	1,871
Total current assets	466,524	354,658
Property and equipment - net	877,818	785,472
Restricted investments	15,485	14,685
Other	16,253	18,508
Total assets	1,376,080	1,173,323
Current Liabilities:
Accounts payable	27,700	36,874
Accrued and other current liabilities	52,812	54,531
Total current liabilities	80,512	91,405
Long-term debt	543,000	379,000
Deferred income taxes	140,532	127,357
Other long-term liabilities	14,968	15,596
Commitments and contingencies (Note 12)
Shareholders' Equity:
Additional paid-in capital	301,533	296,932
Accumulated other comprehensive loss	(211)	(24)
Retained earnings	294,197	261,509
Total shareholders' equity	597,068	559,965
Total liabilities and shareholders' equity	1,376,080	1,173,323
Voting Common Stock [Member]
Shareholders' Equity:
Common stock	291	291
Total shareholders' equity	291	291
Non-Voting Common Stock [Member]
Shareholders' Equity:
Common stock	1,258	1,257
Total shareholders' equity	$ 1,258	$ 1,257