Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Current Assets:
Cash	$ 3,680	$ 2,596
Short-term investments	276,818	289,806
Accounts receivable - net
Trade	117,386	128,662
Other	8,884	9,603
Inventories of spare parts - net	73,033	70,402
Prepaid expenses	10,330	9,259
Deferred income taxes	10,798	10,798
Income taxes receivable	323	540
Total current assets	501,252	521,666
Property and equipment - net	896,565	903,977
Restricted cash and investments	13,038	13,038
Other assets	8,873	9,759
Total assets	1,419,728	1,448,440
Current Liabilities:
Accounts payable	22,054	28,704
Accrued and other current liabilities	27,500	28,346
Total current liabilities	49,554	57,050
Long-term debt:
Revolving credit facility	135,500	134,000
Senior Notes issued March 17, 2014, net of debt issuance costs of $2,441 and $2,753, respectively	497,559	497,247
Deferred income taxes	142,870	151,713
Other long-term liabilities	8,131	8,652
Commitments and contingencies (Note 9)
Shareholders' Equity:
Additional paid-in capital	304,698	304,246
Accumulated other comprehensive income (loss)	(375)	(478)
Retained earnings	280,221	294,441
Total shareholders' equity	586,114	599,778
Total liabilities and shareholders' equity	1,419,728	1,448,440
Voting Common Stock [Member]
Shareholders' Equity:
Common stock	291	291
Non-Voting Common Stock [Member]
Shareholders' Equity:
Common stock	1,279	1,278
Total shareholders' equity	$ 1,279	$ 1,278