Consolidated Balance Sheets - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Current Assets:
Cash	$ 8,770,000	$ 2,596,000
Short-term investments	64,237,000	289,806,000
Accounts receivable – net
Trade	168,153,000	128,662,000
Other	17,826,000	9,603,000
Inventories of spare parts – net	80,881,000	70,402,000
Prepaid expenses	11,475,000	9,259,000
Deferred income taxes	0	10,798,000
Income taxes receivable	1,271,000	540,000
Total current assets	352,613,000	521,666,000
Property and equipment – net	946,765,000	903,977,000
Restricted cash and investments	12,396,000	13,038,000
Other assets	8,741,000	9,759,000
Deferred income taxes	3,309,000	0
Goodwill	61,299,000	0
Intangibles	16,723,000	0
Total assets	1,401,846,000	1,448,440,000
Current Liabilities:
Accounts payable	37,186,000	28,704,000
Accrued and other current liabilities	41,850,000	28,346,000
Total current liabilities	79,036,000	57,050,000
Long-term debt:
Revolving credit facility	117,500,000	134,000,000
Senior Notes issued March 17, 2014, net of unamortized debt issuance costs of $1,506 and $2,753, respectively	498,494,000	497,247,000
Deferred income taxes	86,005,000	151,713,000
Other long-term liabilities	8,157,000	8,652,000
Commitments and contingencies (Note 13)
Shareholders’ Equity:
Additional paid-in capital	308,353,000	304,246,000
Accumulated other comprehensive loss	(280,000)	(478,000)
Retained earnings	303,001,000	294,441,000
Total shareholders’ equity	612,654,000	599,778,000
Total liabilities and shareholders’ equity	1,401,846,000	1,448,440,000
Voting Common Stock [Member]
Shareholders’ Equity:
Common stock	291,000	291,000
Non-Voting Common Stock [Member]
Shareholders’ Equity:
Common stock	1,289,000	1,278,000
Total shareholders’ equity	$ 1,289,000	$ 1,278,000