Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current Assets:
Cash	$ 2,991	$ 2,596
Short-term investments	204,036	289,806
Accounts receivable – net
Trade	140,434	128,662
Other	12,303	9,603
Inventories of spare parts – net	79,245	70,402
Prepaid expenses	12,006	9,259
Deferred income taxes	10,798	10,798
Income taxes receivable	509	540
Total current assets	462,322	521,666
Property and equipment – net	910,327	903,977
Restricted cash and investments	12,396	13,038
Other assets	9,032	9,759
Total assets	1,394,077	1,448,440
Current Liabilities:
Accounts payable	30,607	28,704
Accrued and other current liabilities	33,703	28,346
Total current liabilities	64,310	57,050
Long-term debt:
Revolving credit facility	100,000	134,000
Senior Notes issued March 17, 2014, net of debt issuance costs of $1,818 and $2,753, respectively	498,182	497,247
Deferred income taxes	141,420	151,713
Other long-term liabilities	8,104	8,652
Commitments and contingencies (Note 9)
Shareholders’ Equity:
Additional paid-in capital	307,054	304,246
Accumulated other comprehensive loss	(244)	(478)
Retained earnings	273,671	294,441
Total shareholders’ equity	582,061	599,778
Total liabilities and shareholders’ equity	1,394,077	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