GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017
Current assets:
Cash and cash equivalents	$ 243,912		$ 284,355	$ 334,764		$ 521,375
Short-term investments	26,118		41,461
Accounts receivable, net of allowance	552,737		565,202
Inventories of materials and supplies, net	161,526		158,134
Prepaid expenses and other	63,711		66,398
Total current assets	1,048,004		1,115,550
Investments	60,247		98,696
Property, plant and equipment, net	4,886,948		4,857,382
Goodwill	67,902		64,777
Intangible assets, net	70,531		73,207
Other assets	10,930		5,255
Total assets	6,144,562		6,214,867
Current Liabilities:
Accounts payable	130,721		132,664
Accrued liabilities	242,986		244,504
Total current liabilities	373,707		377,168
Noncurrent Liabilities:
Long-term debt	491,227		493,968
Deferred income taxes	861,440		853,136
Other	84,989		93,606
Noncurrent liabilities - discontinued operations	14,579		14,254
Total noncurrent liabilities	1,452,235		1,454,964
Shareholders' Equity:
Common stock	11,208		11,201
Additional paid-in capital	493,421		500,393
Retained earnings	3,979,708		4,027,779
Accumulated other comprehensive income (loss)	(12,072)		16,550
Treasury stock, at cost, 2,667,837 shares and 3,015,243 shares as of March 31, 2019 and September 30, 2018, respectively	(153,645)		(173,188)
Total shareholders’ equity	4,318,620	$ 4,326,238	4,382,735	$ 4,503,153	$ 4,590,350	$ 4,164,591
Total liabilities and shareholders' equity	6,144,562		6,214,867
Eliminations
Current assets:
Accounts receivable, net of allowance	(183)		(272)
Prepaid expenses and other	(83)		(573)
Total current assets	(266)		(845)
Intercompany receivables	(2,573,930)		(2,480,390)
Investment in subsidiaries	(6,355,360)		(6,153,710)
Total assets	(8,929,556)		(8,634,945)
Current Liabilities:
Accounts payable	(175)		(264)
Accrued liabilities	(91)		(580)
Total current liabilities	(266)		(844)
Noncurrent Liabilities:
Intercompany payables	(2,573,830)		(2,480,290)
Total noncurrent liabilities	(2,573,830)		(2,480,290)
Shareholders' Equity:
Common stock	(100)		(100)
Additional paid-in capital	(53,476)		(53,477)
Retained earnings	(6,310,831)		(6,080,404)
Accumulated other comprehensive income (loss)	8,947		(19,830)
Total shareholders’ equity	(6,355,460)		(6,153,811)
Total liabilities and shareholders' equity	(8,929,556)		(8,634,945)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Accounts receivable, net of allowance	(286)		(29)
Prepaid expenses and other	14,382		20,783
Total current assets	14,096		20,754
Investments	14,460		16,200
Property, plant and equipment, net	46,425		46,859
Intercompany receivables	292,395		161,532
Other assets	317		268
Investment in subsidiaries	6,071,562		5,981,197
Total assets	6,439,255		6,226,810
Current Liabilities:
Accounts payable	84,763		83,819
Accrued liabilities	12,497		43,449
Total current liabilities	97,260		127,268
Noncurrent Liabilities:
Long-term debt	482,543
Deferred income taxes	(3,843)		(7,112)
Intercompany payables	1,523,963		1,701,694
Other	20,712		22,225
Total noncurrent liabilities	2,023,375		1,716,807
Shareholders' Equity:
Common stock	11,208		11,201
Additional paid-in capital	493,421		500,393
Retained earnings	3,979,708		4,027,779
Accumulated other comprehensive income (loss)	(12,072)		16,550
Treasury stock, at cost, 2,667,837 shares and 3,015,243 shares as of March 31, 2019 and September 30, 2018, respectively	(153,645)		(173,188)
Total shareholders’ equity	4,318,620		4,382,735
Total liabilities and shareholders' equity	6,439,255		6,226,810
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	228,460		273,214
Short-term investments	24,441		41,461
Accounts receivable, net of allowance	498,955		499,644
Inventories of materials and supplies, net	129,531		127,154
Prepaid expenses and other	13,550		10,649
Total current assets	894,937		952,122
Investments	45,521		82,496
Property, plant and equipment, net	4,571,458		4,515,077
Intercompany receivables	1,764,216		2,024,652
Other assets	6,061		907
Investment in subsidiaries	283,798		172,513
Total assets	7,565,991		7,747,767
Current Liabilities:
Accounts payable	42,241		43,626
Accrued liabilities	191,895		164,542
Total current liabilities	234,136		208,168
Noncurrent Liabilities:
Long-term debt	8,684		493,968
Deferred income taxes	851,018		834,714
Intercompany payables	236,628		178,759
Other	46,492		48,836
Total noncurrent liabilities	1,142,822		1,556,277
Shareholders' Equity:
Common stock	100		100
Additional paid-in capital	52,437		52,437
Retained earnings	6,145,443		5,910,955
Accumulated other comprehensive income (loss)	(8,947)		19,830
Total shareholders’ equity	6,189,033		5,983,322
Total liabilities and shareholders' equity	7,565,991		7,747,767
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	15,452		11,141
Short-term investments	1,677
Accounts receivable, net of allowance	54,251		65,859
Inventories of materials and supplies, net	31,995		30,980
Prepaid expenses and other	35,862		35,539
Total current assets	139,237		143,519
Investments	266
Property, plant and equipment, net	269,065		295,446
Intercompany receivables	517,319		294,206
Goodwill	67,902		64,777
Intangible assets, net	70,531		73,207
Other assets	4,552		4,080
Total assets	1,068,872		875,235
Current Liabilities:
Accounts payable	3,892		5,483
Accrued liabilities	38,685		37,093
Total current liabilities	42,577		42,576
Noncurrent Liabilities:
Deferred income taxes	14,265		25,534
Intercompany payables	813,239		599,837
Other	17,785		22,545
Noncurrent liabilities - discontinued operations	14,579		14,254
Total noncurrent liabilities	859,868		662,170
Shareholders' Equity:
Additional paid-in capital	1,039		1,040
Retained earnings	165,388		169,449
Total shareholders’ equity	166,427		170,489
Total liabilities and shareholders' equity	$ 1,068,872		$ 875,235