CONDENSED CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Current Assets:
Cash	$ 22,617	$ 934
Short-term investments	135,796	85,916
Accounts receivable - net
Trade	178,424	172,538
Other	2,068	1,182
Inventories of spare parts - net	73,318	71,137
Prepaid expenses	10,317	8,357
Deferred income taxes	12,723	12,723
Income taxes receivable	1,221	1,871
Total current assets	436,484	354,658
Property and equipment - net	822,742	785,472
Restricted investments	15,485	14,685
Other assets	21,794	18,508
Total assets	1,296,505	1,173,323
Current Liabilities:
Accounts payable	33,533	36,874
Accrued and other current liabilities	48,545	54,531
Total current liabilities	82,078	91,405
Long-term debt	500,000	379,000
Deferred income taxes	129,973	127,357
Other long-term liabilities	14,858	15,596
Commitments and contingencies (Note 9)
Shareholders' Equity:
Additional paid-in capital	298,485	296,932
Accumulated other comprehensive loss (income)	6	(24)
Retained earnings	269,556	261,509
Total shareholders' equity	569,596	559,965
Total liabilities and shareholders' equity	1,296,505	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