GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2014
Current assets:
Cash and cash equivalents	$ 521,375	$ 905,561	$ 729,384	$ 360,307
Short-term investments	44,491	44,148
Accounts receivable, net of reserve	477,074	375,169
Inventories	137,204	124,325
Prepaid expenses and other	55,120	78,067
Assets held for sale		45,352
Current assets of discontinued operations	3	64
Total current assets	1,235,267	1,572,686
INVESTMENTS	84,026	84,955
Property, plant and equipment, at cost	5,001,051	5,144,733	5,563,170
Goodwill	51,705	4,718	4,718
Intangible assets, net of amortization	50,785	919
Other assets	17,154	24,008
TOTAL ASSETS	6,439,988	6,832,019
CURRENT LIABILITIES:
Accounts payable	135,628	95,422
Accrued liabilities	208,683	234,639
Current liabilities of discontinued operations	74	59
Total current liabilities	344,385	330,120
NONCURRENT LIABILITIES:
Carrying value of long-term fixed-rate debt	492,902	491,847
Deferred income taxes	1,332,689	1,342,456
Other	101,409	102,781
Noncurrent liabilities of discontinued operations	4,012	3,890
Total noncurrent liabilities	1,931,012	1,940,974
SHAREHOLDERS' EQUITY:
Common stock	11,196	11,140
Additional paid-in capital	487,248	448,452
Retained earnings	3,855,686	4,289,807
Accumulated other comprehensive income (loss), after tax	2,300	(204)
Treasury stock, at cost	(191,839)	(188,270)
Total shareholders' equity	4,164,591	4,560,925	4,895,846	4,891,169
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,439,988	6,832,019
Eliminations
Current assets:
Accounts receivable, net of reserve	(5)	(1,279)
Prepaid expenses and other	(442)	(21,239)
Total current assets	(447)	(22,518)
Intercompany	(2,086,087)	(1,676,409)
Investment in subsidiaries	(5,653,432)	(5,787,831)
TOTAL ASSETS	(7,739,966)	(7,486,758)
CURRENT LIABILITIES:
Accounts payable		(1,274)
Accrued liabilities	(447)	20,234
Total current liabilities	(447)	18,960
NONCURRENT LIABILITIES:
Intercompany	(2,085,987)	(1,717,787)
Total noncurrent liabilities	(2,085,987)	(1,717,787)
SHAREHOLDERS' EQUITY:
Common stock	(100)	(100)
Additional paid-in capital	(53,476)	(48,082)
Retained earnings	(5,593,218)	(5,734,655)
Accumulated other comprehensive income (loss), after tax	(6,738)	(5,094)
Total shareholders' equity	(5,653,532)	(5,787,931)
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	(7,739,966)	(7,486,758)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Cash and cash equivalents	(587)	(955)	(838)	(2,050)
Accounts receivable, net of reserve	766	2
Prepaid expenses and other	12,200	6,928
Total current assets	12,379	5,975
INVESTMENTS	13,853	13,431
Property, plant and equipment, at cost	49,851	59,173
Intercompany	90,885	16,147
Other assets	4,955	233
Investment in subsidiaries	5,470,050	5,579,713
TOTAL ASSETS	5,641,973	5,674,672
CURRENT LIABILITIES:
Accounts payable	82,360	80,000
Accrued liabilities	26,698	1,822
Total current liabilities	109,058	81,822
NONCURRENT LIABILITIES:
Deferred income taxes	(11,201)	(5,930)
Intercompany	1,354,068	1,016,673
Other	25,457	21,182
Total noncurrent liabilities	1,368,324	1,031,925
SHAREHOLDERS' EQUITY:
Common stock	11,196	11,140
Additional paid-in capital	487,248	448,452
Retained earnings	3,855,686	4,289,807
Accumulated other comprehensive income (loss), after tax	2,300	(204)
Treasury stock, at cost	(191,839)	(188,270)
Total shareholders' equity	4,164,591	4,560,925
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	5,641,973	5,674,672
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	508,091	899,028	693,273	329,655
Short-term investments	44,491	44,148
Accounts receivable, net of reserve	411,599	325,325
Inventories	102,470	87,946
Prepaid expenses and other	6,383	20,625
Assets held for sale		18,471
Total current assets	1,073,034	1,395,543
INVESTMENTS	70,173	71,524
Property, plant and equipment, at cost	4,609,144	4,716,736
Intercompany	1,746,662	1,399,323
Other assets	3,839	267
Investment in subsidiaries	183,382	208,118
TOTAL ASSETS	7,686,234	7,791,511
CURRENT LIABILITIES:
Accounts payable	48,679	10,868
Accrued liabilities	148,491	176,985
Total current liabilities	197,170	187,853
NONCURRENT LIABILITIES:
Carrying value of long-term fixed-rate debt	492,902	491,847
Deferred income taxes	1,286,381	1,303,324
Intercompany	210,823	209,276
Other	43,471	36,379
Total noncurrent liabilities	2,033,577	2,040,826
SHAREHOLDERS' EQUITY:
Common stock	100	100
Additional paid-in capital	52,437	47,533
Retained earnings	5,396,212	5,510,105
Accumulated other comprehensive income (loss), after tax	6,738	5,094
Total shareholders' equity	5,455,487	5,562,832
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	7,686,234	7,791,511
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	13,871	7,488	$ 36,949	$ 32,702
Accounts receivable, net of reserve	64,714	51,121
Inventories	34,734	36,379
Prepaid expenses and other	36,979	71,753
Assets held for sale		26,881
Current assets of discontinued operations	3	64
Total current assets	150,301	193,686
Property, plant and equipment, at cost	342,056	368,824
Intercompany	248,540	260,939
Goodwill	51,705	4,718
Intangible assets, net of amortization	50,785	919
Other assets	8,360	23,508
TOTAL ASSETS	851,747	852,594
CURRENT LIABILITIES:
Accounts payable	4,589	5,828
Accrued liabilities	33,941	35,598
Current liabilities of discontinued operations	74	59
Total current liabilities	38,604	41,485
NONCURRENT LIABILITIES:
Deferred income taxes	57,509	45,062
Intercompany	521,096	491,838
Other	32,481	45,220
Noncurrent liabilities of discontinued operations	4,012	3,890
Total noncurrent liabilities	615,098	586,010
SHAREHOLDERS' EQUITY:
Additional paid-in capital	1,039	549
Retained earnings	197,006	224,550
Total shareholders' equity	198,045	225,099
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 851,747	$ 852,594