GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2013
CURRENT ASSETS:
Cash and cash equivalents	$ 905,561	$ 729,384	$ 360,307	$ 435,949
Short-term investments	44,148	45,543
Accounts receivable, net of reserve	375,169	445,948
Inventories	124,325	128,541
Deferred income taxes		17,206
Prepaid expenses and other	78,067	64,475
Assets held for sale	45,352
Current assets of discontinued operations	64	8,097
Total current assets	1,572,686	1,439,194
Investments	84,955	104,354
Property, plant and equipment, net	5,144,733	5,563,170	5,187,587
Other assets	29,645	40,524
TOTAL ASSETS	6,832,019	7,147,242
CURRENT LIABILITIES:
Long-term debt due within one year		39,094
Accounts payable	95,422	108,169
Accrued liabilities	234,639	197,557
Current liabilities of discontinued operations	59	3,377
Total current liabilities	330,120	348,197
NONCURRENT LIABILITIES:
Long-term debt	491,847	492,443
Deferred income taxes	1,342,456	1,295,916
Other	102,781	110,120
Noncurrent liabilities of discontinued operations	3,890	4,720
Total noncurrent liabilities	1,940,974	1,903,199
SHAREHOLDERS' EQUITY:
Common stock	11,140	11,099
Additional paid-in capital	448,452	420,141
Retained earnings	4,289,807	4,648,346
Accumulated other comprehensive income (loss)	(204)	(1,377)
Less treasury stock, 3,322,423 shares in 2016 and 3,219,631 shares in 2015, at cost	(188,270)	(182,363)
Total shareholders' equity	4,560,925	4,895,846	4,891,169	4,446,075
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,832,019	7,147,242
Eliminations
CURRENT ASSETS:
Accounts receivable, net of reserve	(1,279)	(5)
Deferred income taxes		(4,905)
Prepaid expenses and other	(21,239)	(7,903)
Total current assets	(22,518)	(12,813)
Intercompany	(1,676,409)	(1,439,161)
Other assets		(8,153)
Investment in subsidiaries	(5,787,831)	(5,851,341)
TOTAL ASSETS	(7,486,758)	(7,311,468)
CURRENT LIABILITIES:
Accounts payable	(1,274)	(5)
Accrued liabilities	20,234	(11,103)
Total current liabilities	18,960	(11,108)
NONCURRENT LIABILITIES:
Deferred income taxes		(13,058)
Intercompany	(1,717,787)	(1,435,961)
Total noncurrent liabilities	(1,717,787)	(1,449,019)
SHAREHOLDERS' EQUITY:
Common stock	(100)	(100)
Additional paid-in capital	(48,082)	(46,173)
Retained earnings	(5,734,655)	(5,801,212)
Accumulated other comprehensive income (loss)	(5,094)	(3,856)
Total shareholders' equity	(5,787,931)	(5,851,341)
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	(7,486,758)	(7,311,468)
Guarantor/Parent | Reportable Legal Entities
CURRENT ASSETS:
Cash and cash equivalents	(955)	(838)	(2,050)	(202)
Accounts receivable, net of reserve	2	152
Deferred income taxes		2,834
Prepaid expenses and other	6,928	20,018
Total current assets	5,975	22,166
Investments	13,431	12,871
Property, plant and equipment, net	59,173	55,902
Intercompany	16,147	15,875
Other assets	233	8,387
Investment in subsidiaries	5,579,713	5,623,754
TOTAL ASSETS	5,674,672	5,738,955
CURRENT LIABILITIES:
Accounts payable	80,000	80,673
Accrued liabilities	1,822	10,688
Total current liabilities	81,822	91,361
NONCURRENT LIABILITIES:
Deferred income taxes	(5,930)
Intercompany	1,016,673	733,008
Other	21,182	18,740
Total noncurrent liabilities	1,031,925	751,748
SHAREHOLDERS' EQUITY:
Common stock	11,140	11,099
Additional paid-in capital	448,452	420,141
Retained earnings	4,289,807	4,648,346
Accumulated other comprehensive income (loss)	(204)	(1,377)
Less treasury stock, 3,322,423 shares in 2016 and 3,219,631 shares in 2015, at cost	(188,270)	(182,363)
Total shareholders' equity	4,560,925	4,895,846
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	5,674,672	5,738,955
Issuer Subsidiary | Reportable Legal Entities
CURRENT ASSETS:
Cash and cash equivalents	899,028	693,273	329,655	412,596
Short-term investments	44,148	45,543
Accounts receivable, net of reserve	325,325	374,383
Inventories	87,946	88,010
Deferred income taxes		19,277
Prepaid expenses and other	20,625	6,713
Assets held for sale	18,471
Total current assets	1,395,543	1,227,199
Investments	71,524	91,483
Property, plant and equipment, net	4,716,736	5,063,705
Intercompany	1,399,323	1,192,634
Other assets	267	1,389
Investment in subsidiaries	208,118	227,587
TOTAL ASSETS	7,791,511	7,803,997
CURRENT LIABILITIES:
Long-term debt due within one year		39,094
Accounts payable	10,868	20,404
Accrued liabilities	176,985	151,721
Total current liabilities	187,853	211,219
NONCURRENT LIABILITIES:
Long-term debt	491,847	492,443
Deferred income taxes	1,303,324	1,275,428
Intercompany	209,276	186,784
Other	36,379	31,560
Total noncurrent liabilities	2,040,826	1,986,215
SHAREHOLDERS' EQUITY:
Common stock	100	100
Additional paid-in capital	47,533	45,824
Retained earnings	5,510,105	5,556,783
Accumulated other comprehensive income (loss)	5,094	3,856
Total shareholders' equity	5,562,832	5,606,563
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	7,791,511	7,803,997
Non-Guarantor Subsidiaries | Reportable Legal Entities
CURRENT ASSETS:
Cash and cash equivalents	7,488	36,949	$ 32,702	$ 23,555
Accounts receivable, net of reserve	51,121	71,418
Inventories	36,379	40,531
Prepaid expenses and other	71,753	45,647
Assets held for sale	26,881
Current assets of discontinued operations	64	8,097
Total current assets	193,686	202,642
Property, plant and equipment, net	368,824	443,563
Intercompany	260,939	230,652
Other assets	29,145	38,901
TOTAL ASSETS	852,594	915,758
CURRENT LIABILITIES:
Accounts payable	5,828	7,097
Accrued liabilities	35,598	46,251
Current liabilities of discontinued operations	59	3,377
Total current liabilities	41,485	56,725
NONCURRENT LIABILITIES:
Deferred income taxes	45,062	33,546
Intercompany	491,838	516,169
Other	45,220	59,820
Noncurrent liabilities of discontinued operations	3,890	4,720
Total noncurrent liabilities	586,010	614,255
SHAREHOLDERS' EQUITY:
Additional paid-in capital	549	349
Retained earnings	224,550	244,429
Total shareholders' equity	225,099	244,778
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 852,594	$ 915,758