Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Current Assets:
Cash	$ 2,323	$ 2,596
Short-term investments	237,433	289,806
Accounts receivable – net
Trade	133,934	128,662
Other	13,570	9,603
Inventories of spare parts – net	76,155	70,402
Prepaid expenses	13,131	9,259
Deferred income taxes	10,798	10,798
Income taxes receivable	414	540
Total current assets	487,758	521,666
Property and equipment – net	918,134	903,977
Restricted cash and investments	12,396	13,038
Other assets	10,172	9,759
Total assets	1,428,460	1,448,440
Current Liabilities:
Accounts payable	27,791	28,704
Accrued and other current liabilities	33,384	28,346
Total current liabilities	61,175	57,050
Long-term debt:
Revolving credit facility	133,225	134,000
Senior Notes issued March 17, 2014, net of debt issuance costs of $2,129 and $2,753, respectively	497,871	497,247
Deferred income taxes	143,030	151,713
Other long-term liabilities	9,098	8,652
Commitments and contingencies (Note 9)
Shareholders’ Equity:
Additional paid-in capital	305,787	304,246
Accumulated other comprehensive loss	(254)	(478)
Retained earnings	276,948	294,441
Total shareholders’ equity	584,061	599,778
Total liabilities and shareholders’ equity	1,428,460	1,448,440
Voting Common Stock [Member]
Shareholders’ Equity:
Common stock	291	291
Non-Voting Common Stock [Member]
Shareholders’ Equity:
Common stock	1,289	1,278
Total shareholders’ equity	$ 1,289	$ 1,278