Guarantor and Non-Guarantor Financial Information - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Current assets:
Cash and cash equivalents	$ 848,230	$ 729,384	$ 252,732	$ 360,307
Short-term Investments	47,708	45,543
Accounts receivable, net of reserve	373,896	445,948
Inventories	127,242	128,541
Deferred income taxes		17,206
Prepaid expenses and other	71,151	64,475
Current assets of discontinued operations	8,449	8,097
Total current assets	1,476,676	1,439,194
Investments	85,276	104,354
Property, plant and equipment, net	5,530,817	5,563,170
Other assets	37,505	40,524
Total assets	7,130,274	7,147,242
Current liabilities:
Long-term debt due within one year	39,092	39,094
Accounts payable	96,442	108,169
Accrued liabilities	269,946	197,557
Current liabilities of discontinued operations	3,310	3,377
Total current liabilities	408,790	348,197
Noncurrent liabilities:
Long-term debt	492,668	492,443
Deferred income taxes	1,295,601	1,295,916
Other	97,783	110,120
Noncurrent liabilities of discontinued operations	5,139	4,720
Total noncurrent liabilities	1,891,191	1,903,199
Shareholders' equity:
Common stock	$ 11,130	$ 11,099
Preferred stock
Additional paid-in capital	$ 427,590	$ 420,141
Retained earnings	4,589,597	4,648,346
Accumulated other comprehensive income loss	(12,074)	(1,377)
Treasury stock, at cost	(185,950)	(182,363)
Total shareholders' equity	4,830,293	4,895,846
Total liabilities and shareholders' equity	7,130,274	7,147,242
Eliminations
Current assets:
Accounts receivable, net of reserve	(5)	(5)
Deferred income taxes		(4,905)
Prepaid expenses and other	(6,357)	(7,903)
Total current assets	(6,362)	(12,813)
Intercompany	(1,502,600)	(1,439,161)
Other assets	(10,070)	(8,153)
Investment in subsidiaries	(5,850,844)	(5,851,341)
Total assets	(7,369,876)	(7,311,468)
Current liabilities:
Accounts payable	(5)	(5)
Accrued liabilities	(6,357)	(11,103)
Total current liabilities	(6,362)	(11,108)
Noncurrent liabilities:
Deferred income taxes	(10,070)	(13,058)
Intercompany	(1,502,600)	(1,435,961)
Total noncurrent liabilities	(1,512,670)	(1,449,019)
Shareholders' equity:
Common stock	(100)	(100)
Additional paid-in capital	(47,230)	(46,173)
Retained earnings	(5,810,462)	(5,801,212)
Accumulated other comprehensive income loss	6,948	(3,856)
Total shareholders' equity	(5,850,844)	(5,851,341)
Total liabilities and shareholders' equity	(7,369,876)	(7,311,468)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Cash and cash equivalents	(705)	(838)	372	(2,050)
Accounts receivable, net of reserve	(8)	152
Deferred income taxes		2,834
Prepaid expenses and other	26,550	20,018
Total current assets	25,837	22,166
Investments	11,748	12,871
Property, plant and equipment, net	62,282	55,902
Intercompany	15,821	15,875
Other assets	10,304	8,387
Investment in subsidiaries	5,631,533	5,623,754
Total assets	5,757,525	5,738,955
Current liabilities:
Accounts payable	79,879	80,673
Accrued liabilities	11,610	10,688
Total current liabilities	91,489	91,361
Noncurrent liabilities:
Intercompany	818,406	733,008
Other	17,337	18,740
Total noncurrent liabilities	835,743	751,748
Shareholders' equity:
Common stock	11,130	11,099
Additional paid-in capital	427,590	420,141
Retained earnings	4,589,597	4,648,346
Accumulated other comprehensive income loss	(12,074)	(1,377)
Treasury stock, at cost	(185,950)	(182,363)
Total shareholders' equity	4,830,293	4,895,846
Total liabilities and shareholders' equity	5,757,525	5,738,955
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	816,342	693,273	226,815	329,655
Short-term Investments	47,708	45,543
Accounts receivable, net of reserve	309,127	374,383
Inventories	86,889	88,010
Deferred income taxes		19,277
Prepaid expenses and other	7,133	6,713
Total current assets	1,267,199	1,227,199
Investments	73,528	91,483
Property, plant and equipment, net	5,038,560	5,063,705
Intercompany	1,233,222	1,192,634
Other assets	1,023	1,389
Investment in subsidiaries	219,311	227,587
Total assets	7,832,843	7,803,997
Current liabilities:
Long-term debt due within one year	39,092	39,094
Accounts payable	10,922	20,404
Accrued liabilities	218,527	151,721
Total current liabilities	268,541	211,219
Noncurrent liabilities:
Long-term debt	492,668	492,443
Deferred income taxes	1,268,943	1,275,428
Intercompany	164,173	186,784
Other	24,425	31,560
Total noncurrent liabilities	1,950,209	1,986,215
Shareholders' equity:
Common stock	100	100
Additional paid-in capital	46,856	45,824
Retained earnings	5,574,085	5,556,783
Accumulated other comprehensive income loss	(6,948)	3,856
Total shareholders' equity	5,614,093	5,606,563
Total liabilities and shareholders' equity	7,832,843	7,803,997
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	32,593	36,949	$ 25,545	$ 32,702
Accounts receivable, net of reserve	64,782	71,418
Inventories	40,353	40,531
Prepaid expenses and other	43,825	45,647
Current assets of discontinued operations	8,449	8,097
Total current assets	190,002	202,642
Property, plant and equipment, net	429,975	443,563
Intercompany	253,557	230,652
Other assets	36,248	38,901
Total assets	909,782	915,758
Current liabilities:
Accounts payable	5,646	7,097
Accrued liabilities	46,166	46,251
Current liabilities of discontinued operations	3,310	3,377
Total current liabilities	55,122	56,725
Noncurrent liabilities:
Deferred income taxes	36,728	33,546
Intercompany	520,021	516,169
Other	56,021	59,820
Noncurrent liabilities of discontinued operations	5,139	4,720
Total noncurrent liabilities	617,909	614,255
Shareholders' equity:
Additional paid-in capital	374	349
Retained earnings	236,377	244,429
Total shareholders' equity	236,751	244,778
Total liabilities and shareholders' equity	$ 909,782	$ 915,758