Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Current Assets:
Cash	$ 5,852	$ 8,770
Short-term investments	60,317	64,237
Accounts receivable – net
Trade	173,324	168,153
Other	21,008	17,826
Inventories of spare parts – net	77,747	80,881
Prepaid expenses	13,150	11,475
Income taxes receivable	623	1,271
Total current assets	352,021	352,613
Property and equipment – net	933,583	946,765
Restricted cash and investments	12,076	12,396
Other assets	9,605	8,741
Deferred income taxes	3,237	3,309
Goodwill	61,299	61,299
Intangibles	15,946	16,723
Total assets	1,387,767	1,401,846
Current Liabilities:
Revolving Credit Facility	122,220	0
Senior Notes issued March 17, 2014, net of debt issuance costs of $1,229	498,771	0
Accounts payable	50,645	37,186
Accrued and other current liabilities	40,626	41,850
Total current liabilities	712,262	79,036
Long-term debt:
Revolving credit facility	0	117,500
Senior Notes issued March 17, 2014, net of debt issuance costs of $1,506	0	498,494
Deferred income taxes	78,605	86,005
Other long-term liabilities	5,335	8,157
Commitments and contingencies (Note 9)
Shareholders’ Equity:
Additional paid-in capital	310,433	308,353
Accumulated other comprehensive income (loss)	624	(280)
Retained earnings	278,926	303,001
Total shareholders’ equity	591,565	612,654
Total liabilities and shareholders’ equity	1,387,767	1,401,846
Voting Common Stock [Member]
Shareholders’ Equity:
Common stock	291	291
Non-Voting Common Stock [Member]
Shareholders’ Equity:
Common stock	1,291	1,289
Total shareholders’ equity	$ 1,291	$ 1,289