Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Current Assets:
Cash	$ 8,567	$ 2,407
Short-term investments	289,235	284,523
Accounts receivable - net
Trade	137,031	138,309
Other	11,381	6,469
Inventories of spare parts - net	69,424	69,491
Prepaid expenses	7,802	8,951
Deferred income taxes	10,379	10,379
Income taxes receivable	709	761
Total current assets	534,528	521,290
Property and equipment - net	893,235	883,529
Restricted investments	13,038	15,336
Other assets	6,317	6,178
Total assets	1,447,118	1,426,333
Current Liabilities:
Accounts payable	23,462	31,373
Accrued and other current liabilities	40,343	44,759
Total current liabilities	63,805	76,132
Long-term debt:
Revolving credit facility	95,000	57,500
Senior Notes issued March 17, 2014, net of debt issuance costs of $3,376 and $3,999, respectively	496,624	496,001
Deferred income taxes	150,464	153,645
Other long-term liabilities	15,879	16,057
Commitments and contingencies (Note 9)
Shareholders' Equity:
Additional paid-in capital	307,266	304,884
Accumulated other comprehensive gain (loss)	45	(567)
Retained earnings	316,466	321,121
Total shareholders' equity	625,346	626,998
Total liabilities and shareholders' equity	1,447,118	1,426,333
Voting Common Stock [Member]
Shareholders' Equity:
Common stock	291	291
Non-Voting Common Stock [Member]
Shareholders' Equity:
Common stock	1,278	1,269
Total shareholders' equity	$ 1,278	$ 1,269