Guarantor and Non-Guarantor Financial Information - Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014
Current assets:
Cash and cash equivalents	$ 898,013	$ 729,384	$ 719,785	$ 360,307
Short-term investments	45,526	45,543
Accounts receivable, net of reserve	330,726	445,948
Inventories	129,649	128,541
Deferred income taxes		17,206
Prepaid expenses and other	56,233	64,475
Current assets of discontinued operations	230	8,097
Total current assets	1,460,377	1,439,194
Investments	83,363	104,354
Property, plant and equipment, net	5,446,352	5,563,170
Other assets	35,013	40,524
Total assets	7,025,105	7,147,242
Current liabilities:
Long-term debt due within one year	39,144	39,094
Accounts payable	91,437	108,169
Accrued liabilities	240,665	197,557
Current liabilities of discontinued operations	82	3,377
Total current liabilities	371,328	348,197
Noncurrent liabilities:
Long-term debt	492,919	492,443
Deferred income taxes	1,278,664	1,295,916
Other	95,984	110,120
Noncurrent liabilities of discontinued operations	4,110	4,720
Total noncurrent liabilities	1,871,677	1,903,199
Shareholders' equity:
Common stock	11,136	11,099
Additional paid-in capital	436,117	420,141
Retained earnings	4,536,047	4,648,346
Accumulated other comprehensive loss	(13,214)	(1,377)
Treasury stock, at cost	(187,986)	(182,363)
Total shareholders' equity	4,782,100	4,895,846
Total liabilities and shareholders' equity	7,025,105	7,147,242
Eliminations
Current assets:
Accounts receivable, net of reserve		(5)
Deferred income taxes		(4,905)
Prepaid expenses and other	(16,129)	(7,903)
Total current assets	(16,129)	(12,813)
Intercompany	(1,588,059)	(1,439,161)
Other assets	(10,758)	(8,153)
Investment in subsidiaries	(5,864,744)	(5,851,341)
Total assets	(7,479,690)	(7,311,468)
Current liabilities:
Accounts payable		(5)
Accrued liabilities	(16,129)	(11,103)
Total current liabilities	(16,129)	(11,108)
Noncurrent liabilities:
Deferred income taxes	(10,758)	(13,058)
Intercompany	(1,587,959)	(1,435,961)
Total noncurrent liabilities	(1,598,717)	(1,449,019)
Shareholders' equity:
Common stock	(100)	(100)
Additional paid-in capital	(47,225)	(46,173)
Retained earnings	(5,825,714)	(5,801,212)
Accumulated other comprehensive loss	8,195	(3,856)
Total shareholders' equity	(5,864,844)	(5,851,341)
Total liabilities and shareholders' equity	(7,479,690)	(7,311,468)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Cash and cash equivalents	(1,900)	(838)	(1,949)	(2,050)
Accounts receivable, net of reserve	(13)	152
Deferred income taxes		2,834
Prepaid expenses and other	25,867	20,018
Total current assets	23,954	22,166
Investments	12,205	12,871
Property, plant and equipment, net	64,190	55,902
Intercompany	15,144	15,875
Other assets	10,992	8,387
Investment in subsidiaries	5,653,326	5,623,754
Total assets	5,779,811	5,738,955
Current liabilities:
Accounts payable	78,165	80,673
Accrued liabilities	11,388	10,688
Total current liabilities	89,553	91,361
Noncurrent liabilities:
Intercompany	890,370	733,008
Other	17,788	18,740
Total noncurrent liabilities	908,158	751,748
Shareholders' equity:
Common stock	11,136	11,099
Additional paid-in capital	436,117	420,141
Retained earnings	4,536,047	4,648,346
Accumulated other comprehensive loss	(13,214)	(1,377)
Treasury stock, at cost	(187,986)	(182,363)
Total shareholders' equity	4,782,100	4,895,846
Total liabilities and shareholders' equity	5,779,811	5,738,955
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	885,570	693,273	695,343	329,655
Short-term investments	45,526	45,543
Accounts receivable, net of reserve	270,387	374,383
Inventories	88,799	88,010
Deferred income taxes		19,277
Prepaid expenses and other	3,142	6,713
Total current assets	1,293,424	1,227,199
Investments	71,158	91,483
Property, plant and equipment, net	4,962,375	5,063,705
Intercompany	1,310,646	1,192,634
Other assets	478	1,389
Investment in subsidiaries	211,418	227,587
Total assets	7,849,499	7,803,997
Current liabilities:
Long-term debt due within one year	39,144	39,094
Accounts payable	10,529	20,404
Accrued liabilities	193,249	151,721
Total current liabilities	242,922	211,219
Noncurrent liabilities:
Long-term debt	492,919	492,443
Deferred income taxes	1,267,164	1,275,428
Intercompany	183,734	186,784
Other	27,204	31,560
Total noncurrent liabilities	1,971,021	1,986,215
Shareholders' equity:
Common stock	100	100
Additional paid-in capital	46,832	45,824
Retained earnings	5,596,819	5,556,783
Accumulated other comprehensive loss	(8,195)	3,856
Total shareholders' equity	5,635,556	5,606,563
Total liabilities and shareholders' equity	7,849,499	7,803,997
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	14,343	36,949	$ 26,391	$ 32,702
Accounts receivable, net of reserve	60,352	71,418
Inventories	40,850	40,531
Prepaid expenses and other	43,353	45,647
Current assets of discontinued operations	230	8,097
Total current assets	159,128	202,642
Property, plant and equipment, net	419,787	443,563
Intercompany	262,269	230,652
Other assets	34,301	38,901
Total assets	875,485	915,758
Current liabilities:
Accounts payable	2,743	7,097
Accrued liabilities	52,157	46,251
Current liabilities of discontinued operations	82	3,377
Total current liabilities	54,982	56,725
Noncurrent liabilities:
Deferred income taxes	22,258	33,546
Intercompany	513,855	516,169
Other	50,992	59,820
Noncurrent liabilities of discontinued operations	4,110	4,720
Total noncurrent liabilities	591,215	614,255
Shareholders' equity:
Additional paid-in capital	393	349
Retained earnings	228,895	244,429
Total shareholders' equity	229,288	244,778
Total liabilities and shareholders' equity	$ 875,485	$ 915,758