GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Current assets:
Cash and cash equivalents	$ 228,462	$ 284,355	$ 383,664	$ 521,375
Short-term investments	41,072	41,461
Accounts receivable, net of allowance	545,731	565,202
Inventories of materials and supplies, net	159,992	158,134
Prepaid expenses and other	67,490	66,398
Total current assets	1,042,747	1,115,550
Investments	54,731	98,696
Property, plant and equipment, net	4,900,339	4,857,382
Goodwill	67,902	64,777
Intangible assets, net	71,969	73,207
Other assets	6,653	5,255
Total assets	6,144,341	6,214,867
Current Liabilities:
Accounts payable	141,406	132,664
Accrued liabilities	244,507	244,504
Total current liabilities	385,913	377,168
Noncurrent Liabilities:
Long-term debt	490,805	493,968
Deferred income taxes	853,187	853,136
Other	84,565	93,606
Noncurrent liabilities - discontinued operations	3,633	14,254
Total noncurrent liabilities	1,432,190	1,454,964
Shareholders' Equity:
Common stock	11,208	11,201
Additional paid-in capital	484,122	500,393
Retained earnings	3,997,283	4,027,779
Accumulated other comprehensive income (loss)	(12,296)	16,550
Treasury stock, at cost, 2,675,372 shares and 3,015,243 shares as of December 31, 2018 and September 30, 2018, respectively	(154,079)	(173,188)
Total shareholders’ equity	4,326,238	4,382,735
Total liabilities and shareholders' equity	6,144,341	6,214,867
Eliminations
Current assets:
Accounts receivable, net of allowance	(368)	(272)
Prepaid expenses and other	(327)	(573)
Total current assets	(695)	(845)
Intercompany receivables	(2,448,567)	(2,480,390)
Investment in subsidiaries	(6,189,627)	(6,153,710)
Total assets	(8,638,889)	(8,634,945)
Current Liabilities:
Accounts payable	(360)	(264)
Accrued liabilities	(335)	(580)
Total current liabilities	(695)	(844)
Noncurrent Liabilities:
Intercompany payables	(2,448,467)	(2,480,290)
Total noncurrent liabilities	(2,448,467)	(2,480,290)
Shareholders' Equity:
Common stock	(100)	(100)
Additional paid-in capital	(53,476)	(53,477)
Retained earnings	(6,145,244)	(6,080,404)
Accumulated other comprehensive income (loss)	9,094	(19,830)
Total shareholders’ equity	(6,189,726)	(6,153,811)
Total liabilities and shareholders' equity	(8,638,888)	(8,634,945)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Accounts receivable, net of allowance	(299)	(29)
Inventories of materials and supplies, net	(1)
Prepaid expenses and other	14,804	20,783
Total current assets	14,504	20,754
Investments	14,957	16,200
Property, plant and equipment, net	47,383	46,859
Intercompany receivables	282,543	161,532
Other assets	318	268
Investment in subsidiaries	6,003,703	5,981,197
Total assets	6,363,408	6,226,810
Current Liabilities:
Accounts payable	83,631	83,819
Accrued liabilities	20,733	43,449
Total current liabilities	104,364	127,268
Noncurrent Liabilities:
Long-term debt	482,126
Deferred income taxes	(3,437)	(7,112)
Intercompany payables	1,432,156	1,701,694
Other	21,960	22,225
Total noncurrent liabilities	1,932,805	1,716,807
Shareholders' Equity:
Common stock	11,208	11,201
Additional paid-in capital	484,122	500,393
Retained earnings	3,997,283	4,027,779
Accumulated other comprehensive income (loss)	(12,296)	16,550
Treasury stock, at cost, 2,675,372 shares and 3,015,243 shares as of December 31, 2018 and September 30, 2018, respectively	(154,079)	(173,188)
Total shareholders’ equity	4,326,238	4,382,735
Total liabilities and shareholders' equity	6,363,407	6,226,810
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	208,219	273,214
Short-term investments	41,072	41,461
Accounts receivable, net of allowance	493,770	499,644
Inventories of materials and supplies, net	129,389	127,154
Prepaid expenses and other	12,626	10,649
Total current assets	885,076	952,122
Investments	39,651	82,496
Property, plant and equipment, net	4,577,824	4,515,077
Intercompany receivables	1,658,200	2,024,652
Other assets	1,266	907
Investment in subsidiaries	185,924	172,513
Total assets	7,347,941	7,747,767
Current Liabilities:
Accounts payable	51,394	43,626
Accrued liabilities	192,779	164,542
Total current liabilities	244,173	208,168
Noncurrent Liabilities:
Long-term debt	8,679	493,968
Deferred income taxes	835,673	834,714
Intercompany payables	208,841	178,759
Other	40,384	48,836
Total noncurrent liabilities	1,093,577	1,556,277
Shareholders' Equity:
Common stock	100	100
Additional paid-in capital	52,437	52,437
Retained earnings	5,966,748	5,910,955
Accumulated other comprehensive income (loss)	(9,094)	19,830
Total shareholders’ equity	6,010,191	5,983,322
Total liabilities and shareholders' equity	7,347,941	7,747,767
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	20,243	11,141
Accounts receivable, net of allowance	52,628	65,859
Inventories of materials and supplies, net	30,604	30,980
Prepaid expenses and other	40,387	35,539
Total current assets	143,862	143,519
Investments	123
Property, plant and equipment, net	275,132	295,446
Intercompany receivables	507,824	294,206
Goodwill	67,902	64,777
Intangible assets, net	71,969	73,207
Other assets	5,069	4,080
Total assets	1,071,881	875,235
Current Liabilities:
Accounts payable	6,741	5,483
Accrued liabilities	31,330	37,093
Total current liabilities	38,071	42,576
Noncurrent Liabilities:
Deferred income taxes	20,951	25,534
Intercompany payables	807,470	599,837
Other	22,221	22,545
Noncurrent liabilities - discontinued operations	3,633	14,254
Total noncurrent liabilities	854,275	662,170
Shareholders' Equity:
Additional paid-in capital	1,039	1,040
Retained earnings	178,496	169,449
Total shareholders’ equity	179,535	170,489
Total liabilities and shareholders' equity	$ 1,071,881	$ 875,235