Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Current Assets:
Cash	$ 2,596	$ 2,407
Short-term investments	289,806	284,523
Accounts receivable - net
Trade	128,662	138,309
Other	9,603	6,469
Inventories of spare parts - net	70,402	69,491
Prepaid expenses	9,259	8,951
Deferred income taxes	10,798	10,379
Income taxes receivable	540	761
Total current assets	521,666	521,290
Property and equipment - net	903,977	883,529
Restricted cash and investments	13,038	15,336
Other	9,759	6,178
Total assets	1,448,440	1,426,333
Current Liabilities:
Accounts payable	28,704	31,373
Accrued and other current liabilities	28,346	44,759
Total current liabilities	57,050	76,132
Long-term debt:
Revolving credit facility	134,000	57,500
Senior Notes issued March 17, 2014, net of unamortized debt issuance costs of $2,753 and $3,999, respectively	497,247	496,001
Deferred income taxes	151,713	153,645
Other long-term liabilities	8,652	16,057
Commitments and contingencies (Note 12)
Shareholders' Equity:
Additional paid-in capital	304,246	304,884
Accumulated other comprehensive income (loss)	(478)	(567)
Retained earnings	294,441	321,121
Total shareholders' equity	599,778	626,998
Total liabilities and shareholders' equity	1,448,440	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