Condensed Consolidating Financial Information - Guarantor Subsidiaries - Condensed Consolidating Balance Sheets (Detail) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Current Assets:
Cash		$ 3,680	$ 2,596	$ 6,536	$ 2,407
Short-term investments		276,818	289,806
Accounts receivable - net		126,270	138,265
Inventories of spare parts - net		73,033	70,402
Prepaid expenses		10,330	9,259
Deferred income taxes		10,798	10,798
Income taxes receivable		323	540
Total current assets		501,252	521,666
Property and equipment - net		896,565	903,977
Restricted cash and investments		13,038	13,038
Other assets		8,873	9,759
Total assets		1,419,728	1,448,440
Current Liabilities:
Accounts payable		22,054	28,704
Accrued and other current liabilities		27,500	28,346
Total current liabilities		49,554	57,050
Long-term debt		633,059	631,247
Deferred income taxes and other long-term liabilities		151,001	160,365
Shareholders' Equity:
Common stock and paid-in capital		306,268	305,815
Accumulated other comprehensive income (loss)		(375)	(478)
Retained earnings		280,221	294,441
Total shareholders' equity		586,114	599,778	619,537	626,998
Total liabilities and shareholders' equity		1,419,728	1,448,440
Parent Company Only (issuer) [Member]
Current Assets:
Cash		51	36	123	46
Short-term investments		276,818	289,806
Accounts receivable - net		65,044	71,458
Inventories of spare parts - net		64,698	61,834
Prepaid expenses		8,144	6,990
Deferred income taxes		10,798	10,798
Income taxes receivable		341	558
Total current assets		425,894	441,480
Investment in subsidiaries and others		361,420	353,160
Property and equipment - net		581,990	589,104
Restricted cash and investments		13,023	13,023
Other assets		7,819	8,660
Total assets		1,390,146	1,405,427
Current Liabilities:
Accounts payable		18,007	22,744
Accrued and other current liabilities		18,034	18,725
Intercompany payable		69,862	57,904
Total current liabilities		105,903	99,373
Long-term debt		633,059	631,247
Deferred income taxes and other long-term liabilities		65,070	75,029
Shareholders' Equity:
Common stock and paid-in capital		306,268	305,815
Accumulated other comprehensive income (loss)		(375)	(478)
Retained earnings		280,221	294,441
Total shareholders' equity		586,114	599,778
Total liabilities and shareholders' equity		1,390,146	1,405,427
Guarantor Subsidiaries [Member]
Current Assets:
Cash	[1]	3,629	2,560	$ 6,413	$ 2,361
Accounts receivable - net	[1]	61,226	66,807
Intercompany receivable	[1]	69,862	57,904
Inventories of spare parts - net	[1]	8,335	8,568
Prepaid expenses	[1]	2,186	2,269
Income taxes receivable	[1]	(18)	(18)
Total current assets	[1]	145,220	138,090
Property and equipment - net	[1]	314,575	314,873
Restricted cash and investments	[1]	15	15
Other assets	[1]	1,054	1,099
Total assets	[1]	460,864	454,077
Current Liabilities:
Accounts payable	[1]	4,047	5,960
Accrued and other current liabilities	[1]	9,466	9,621
Total current liabilities	[1]	13,513	15,581
Deferred income taxes and other long-term liabilities	[1]	85,931	85,336
Shareholders' Equity:
Common stock and paid-in capital	[1]	79,326	79,191
Retained earnings	[1]	282,094	273,969
Total shareholders' equity	[1]	361,420	353,160
Total liabilities and shareholders' equity	[1]	460,864	454,077
Eliminations [Member]
Current Assets:
Intercompany receivable		(69,862)	(57,904)
Total current assets		(69,862)	(57,904)
Investment in subsidiaries and others		(361,420)	(353,160)
Total assets		(431,282)	(411,064)
Current Liabilities:
Intercompany payable		(69,862)	(57,904)
Total current liabilities		(69,862)	(57,904)
Shareholders' Equity:
Common stock and paid-in capital		(79,326)	(79,191)
Retained earnings		(282,094)	(273,969)
Total shareholders' equity		(361,062)	(353,160)
Total liabilities and shareholders' equity		$ (431,282)	$ (411,064)

[1] Foreign subsidiaries represent minor subsidiaries and are included in the guarantor subsidiaries' amounts.