Guarantor and Non-Guarantor Financial Information - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014
Current assets:
Cash and cash equivalents	$ 907,032	$ 729,384	$ 774,540	$ 360,307
Short-term investments	49,565	45,543
Accounts receivable, net of reserve	351,317	445,948
Inventories	128,885	128,541
Prepaid expenses and other	79,021	64,475
Assets held for sale	21,772
Deferred income taxes		17,206
Current assets of discontinued operations	80	8,097
Total current assets	1,537,672	1,439,194
Investments	99,898	104,354
Property, plant and equipment, net	5,306,434	5,563,170
Other assets	32,515	40,524
Total assets	6,976,519	7,147,242
Current liabilities:
Long-term debt due within one year	39,234	39,094
Accounts payable	92,692	108,169
Accrued liabilities	194,645	197,557
Current liabilities of discontinued operations	38	3,377
Total current liabilities	326,609	348,197
Noncurrent liabilities:
Long-term debt	493,150	492,443
Deferred income taxes	1,358,093	1,295,916
Other	93,221	110,120
Noncurrent liabilities of discontinued operations	3,984	4,720
Total noncurrent liabilities	1,948,448	1,903,199
Shareholders' equity:
Common stock	11,138	11,099
Additional paid-in capital	442,883	420,141
Retained earnings	4,438,748	4,648,346
Accumulated other comprehensive loss	(3,156)	(1,377)
Treasury stock, at cost	(188,151)	(182,363)
Total shareholders' equity	4,701,462	4,895,846
Total liabilities and shareholders' equity	6,976,519	7,147,242
Eliminations
Current assets:
Accounts receivable, net of reserve	(871)	(5)
Prepaid expenses and other	(2,017)	(7,903)
Deferred income taxes		(4,905)
Total current assets	(2,888)	(12,813)
Intercompany	(1,707,082)	(1,439,161)
Other assets	(10,479)	(8,153)
Investment in subsidiaries	(5,849,057)	(5,851,341)
Total assets	(7,569,506)	(7,311,468)
Current liabilities:
Accounts payable	(856)	(5)
Accrued liabilities	(2,032)	(11,103)
Total current liabilities	(2,888)	(11,108)
Noncurrent liabilities:
Deferred income taxes	(11,479)	(13,058)
Intercompany	(1,705,982)	(1,435,961)
Total noncurrent liabilities	(1,717,461)	(1,449,019)
Shareholders' equity:
Common stock	(100)	(100)
Additional paid-in capital	(47,919)	(46,173)
Retained earnings	(5,799,382)	(5,801,212)
Accumulated other comprehensive loss	(1,756)	(3,856)
Total shareholders' equity	(5,849,157)	(5,851,341)
Total liabilities and shareholders' equity	(7,569,506)	(7,311,468)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Cash and cash equivalents	(920)	(838)	(877)	(2,050)
Accounts receivable, net of reserve	39,698	152
Prepaid expenses and other	7,997	20,018
Deferred income taxes		2,834
Total current assets	46,775	22,166
Investments	12,848	12,871
Property, plant and equipment, net	61,989	55,902
Intercompany	15,982	15,875
Other assets	10,713	8,387
Investment in subsidiaries	5,645,245	5,623,754
Total assets	5,793,552	5,738,955
Current liabilities:
Accounts payable	80,926	80,673
Accrued liabilities	20,936	10,688
Total current liabilities	101,862	91,361
Noncurrent liabilities:
Intercompany	971,750	733,008
Other	18,478	18,740
Total noncurrent liabilities	990,228	751,748
Shareholders' equity:
Common stock	11,138	11,099
Additional paid-in capital	442,883	420,141
Retained earnings	4,438,748	4,648,346
Accumulated other comprehensive loss	(3,156)	(1,377)
Treasury stock, at cost	(188,151)	(182,363)
Total shareholders' equity	4,701,462	4,895,846
Total liabilities and shareholders' equity	5,793,552	5,738,955
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	892,476	693,273	755,070	329,655
Short-term investments	49,565	45,543
Accounts receivable, net of reserve	269,495	374,383
Inventories	88,438	88,010
Prepaid expenses and other	33,403	6,713
Assets held for sale	21,772
Deferred income taxes		19,277
Total current assets	1,355,149	1,227,199
Investments	87,050	91,483
Property, plant and equipment, net	4,840,172	5,063,705
Intercompany	1,426,888	1,192,634
Other assets	1,019	1,389
Investment in subsidiaries	203,812	227,587
Total assets	7,914,090	7,803,997
Current liabilities:
Long-term debt due within one year	39,234	39,094
Accounts payable	8,718	20,404
Accrued liabilities	138,267	151,721
Total current liabilities	186,219	211,219
Noncurrent liabilities:
Long-term debt	493,150	492,443
Deferred income taxes	1,321,148	1,275,428
Intercompany	257,947	186,784
Other	28,455	31,560
Total noncurrent liabilities	2,100,700	1,986,215
Shareholders' equity:
Common stock	100	100
Additional paid-in capital	47,372	45,824
Retained earnings	5,577,943	5,556,783
Accumulated other comprehensive loss	1,756	3,856
Total shareholders' equity	5,627,171	5,606,563
Total liabilities and shareholders' equity	7,914,090	7,803,997
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	15,476	36,949	$ 20,347	$ 32,702
Accounts receivable, net of reserve	42,995	71,418
Inventories	40,447	40,531
Prepaid expenses and other	39,638	45,647
Current assets of discontinued operations	80	8,097
Total current assets	138,636	202,642
Property, plant and equipment, net	404,273	443,563
Intercompany	264,212	230,652
Other assets	31,262	38,901
Total assets	838,383	915,758
Current liabilities:
Accounts payable	3,904	7,097
Accrued liabilities	37,474	46,251
Current liabilities of discontinued operations	38	3,377
Total current liabilities	41,416	56,725
Noncurrent liabilities:
Deferred income taxes	48,424	33,546
Intercompany	476,285	516,169
Other	46,288	59,820
Noncurrent liabilities of discontinued operations	3,984	4,720
Total noncurrent liabilities	574,981	614,255
Shareholders' equity:
Additional paid-in capital	547	349
Retained earnings	221,439	244,429
Total shareholders' equity	221,986	244,778
Total liabilities and shareholders' equity	$ 838,383	$ 915,758