GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Current assets:
Cash and cash equivalents	$ 284,355	$ 521,375	$ 905,561	$ 729,384
Short-term investments	41,461	44,491
Accounts receivable, net of allowance	565,202	477,074
Inventories of materials and supplies, net	158,134	137,204
Prepaid expenses and other	66,398	55,123
Total current assets	1,115,550	1,235,267
Investments	98,696	84,026
Property, plant and equipment, net	4,857,382	5,001,051	5,144,733
Goodwill	64,777	51,705	4,718
Intangible assets, net	73,207	50,785
Other assets	5,255	17,154
Total Assets	6,214,867	6,439,988
Current Liabilities:
Accounts payable	132,664	135,628
Accrued liabilities	244,504	208,757
Total current liabilities	377,168	344,385
Noncurrent Liabilities:
Long-term debt	493,968	492,902
Deferred income taxes	853,136	1,332,689
Other	93,606	101,409
Noncurrent liabilities - discontinued operations	14,254	4,012
Total noncurrent liabilities	1,454,964	1,931,012
Shareholders' Equity:
Common stock	11,201	11,196
Additional paid-in capital	500,393	487,248
Retained earnings	4,027,779	3,855,686
Accumulated other comprehensive income (loss)	16,550	2,300
Treasury stock, at cost, 3,015,243 shares and 3,352,828 shares as of September 30, 2018 and 2017, respectively	(173,188)	(191,839)
Total shareholders' equity	4,382,735	4,164,591	4,560,925	4,895,846
Total liabilities and shareholders' equity	6,214,867	6,439,988
Eliminations
Current assets:
Accounts receivable, net of allowance	(272)	(5)
Prepaid expenses and other	(573)	(442)
Total current assets	(845)	(447)
Intercompany receivables	(2,480,390)	(2,086,087)
Investment in subsidiaries	(6,153,710)	(5,653,432)
Total Assets	(8,634,945)	(7,739,966)
Current Liabilities:
Accounts payable	(264)
Accrued liabilities	(580)	(447)
Total current liabilities	(844)	(447)
Noncurrent Liabilities:
Intercompany payables	(2,480,290)	(2,085,987)
Total noncurrent liabilities	(2,480,290)	(2,085,987)
Shareholders' Equity:
Common stock	(100)	(100)
Additional paid-in capital	(53,477)	(53,476)
Retained earnings	(6,080,404)	(5,593,218)
Accumulated other comprehensive income (loss)	(19,830)	(6,738)
Total shareholders' equity	(6,153,811)	(5,653,532)
Total liabilities and shareholders' equity	(8,634,945)	(7,739,966)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Accounts receivable, net of allowance	(29)	766
Prepaid expenses and other	20,783	12,200
Total current assets	20,754	12,966
Investments	16,200	13,853
Property, plant and equipment, net	46,859	49,851
Intercompany receivables	161,532	90,885
Other assets	268	4,955
Investment in subsidiaries	5,981,197	5,470,050
Total Assets	6,226,810	5,642,560
Current Liabilities:
Accounts payable	83,819	82,947
Accrued liabilities	43,449	26,698
Total current liabilities	127,268	109,645
Noncurrent Liabilities:
Deferred income taxes	(7,112)	(11,201)
Intercompany payables	1,701,694	1,354,068
Other	22,225	25,457
Total noncurrent liabilities	1,716,807	1,368,324
Shareholders' Equity:
Common stock	11,201	11,196
Additional paid-in capital	500,393	487,248
Retained earnings	4,027,779	3,855,686
Accumulated other comprehensive income (loss)	16,550	2,300
Treasury stock, at cost, 3,015,243 shares and 3,352,828 shares as of September 30, 2018 and 2017, respectively	(173,188)	(191,839)
Total shareholders' equity	4,382,735	4,164,591
Total liabilities and shareholders' equity	6,226,810	5,642,560
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	273,214	507,504	898,073	692,435
Short-term investments	41,461	44,491
Accounts receivable, net of allowance	499,644	411,599
Inventories of materials and supplies, net	127,154	102,470
Prepaid expenses and other	10,649	6,383
Total current assets	952,122	1,072,447
Investments	82,496	70,173
Property, plant and equipment, net	4,515,077	4,609,144
Intercompany receivables	2,024,652	1,746,662
Other assets	907	3,839
Investment in subsidiaries	172,513	183,382
Total Assets	7,747,767	7,685,647
Current Liabilities:
Accounts payable	43,626	48,092
Accrued liabilities	164,542	148,491
Total current liabilities	208,168	196,583
Noncurrent Liabilities:
Long-term debt	493,968	492,902
Deferred income taxes	834,714	1,286,381
Intercompany payables	178,759	210,823
Other	48,836	43,471
Total noncurrent liabilities	1,556,277	2,033,577
Shareholders' Equity:
Common stock	100	100
Additional paid-in capital	52,437	52,437
Retained earnings	5,910,955	5,396,212
Accumulated other comprehensive income (loss)	19,830	6,738
Total shareholders' equity	5,983,322	5,455,487
Total liabilities and shareholders' equity	7,747,767	7,685,647
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	11,141	13,871	$ 7,488	$ 36,949
Accounts receivable, net of allowance	65,859	64,714
Inventories of materials and supplies, net	30,980	34,734
Prepaid expenses and other	35,539	36,982
Total current assets	143,519	150,301
Property, plant and equipment, net	295,446	342,056
Intercompany receivables	294,206	248,540
Goodwill	64,777	51,705
Intangible assets, net	73,207	50,785
Other assets	4,080	8,360
Total Assets	875,235	851,747
Current Liabilities:
Accounts payable	5,483	4,589
Accrued liabilities	37,093	34,015
Total current liabilities	42,576	38,604
Noncurrent Liabilities:
Deferred income taxes	25,534	57,509
Intercompany payables	599,837	521,096
Other	22,545	32,481
Noncurrent liabilities - discontinued operations	14,254	4,012
Total noncurrent liabilities	662,170	615,098
Shareholders' Equity:
Additional paid-in capital	1,040	1,039
Retained earnings	169,449	197,006
Total shareholders' equity	170,489	198,045
Total liabilities and shareholders' equity	$ 875,235	$ 851,747