Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Current Assets:
Cash	$ 2,639	$ 2,407
Short-term investments	289,520	284,523
Accounts receivable - net
Trade	131,295	138,309
Other	11,180	6,469
Inventories of spare parts - net	69,409	69,491
Prepaid expenses	6,976	8,951
Deferred income taxes	10,379	10,379
Income taxes receivable	863	761
Total current assets	522,261	521,290
Property and equipment - net	916,560	883,529
Restricted cash and investments	13,038	15,336
Other assets	7,334	6,178
Total assets	1,459,193	1,426,333
Current Liabilities:
Accounts payable	20,843	31,373
Accrued and other current liabilities	32,178	44,759
Total current liabilities	53,021	76,132
Long-term debt:
Revolving credit facility	132,400	57,500
Senior Notes issued March 17, 2014, net of debt issuance costs of $3,064 and $3,999, respectively	496,936	496,001
Deferred income taxes	147,058	153,645
Other long-term liabilities	8,291	16,057
Commitments and contingencies (Note 9)
Shareholders' Equity:
Additional paid-in capital	308,690	304,884
Accumulated other comprehensive income (loss)	(271)	(567)
Retained earnings	311,499	321,121
Total shareholders' equity	621,487	626,998
Total liabilities and shareholders' equity	1,459,193	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