Guarantor and Non-Guarantor Financial Information - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016
Current assets:
Cash and cash equivalents	$ 306,426	$ 521,375	$ 572,787	$ 905,561
Short-term investments	44,279	44,491
Accounts receivable, net of allowance	565,321	477,074
Inventories	152,109	137,204
Prepaid expenses and other	65,343	55,120
Current assets - discontinued operations		3
Total current assets	1,133,478	1,235,267
Investments	92,702	84,026
Property, plant and equipment, net	4,883,378	5,001,051
Goodwill	69,496	51,705
Intangible assets, net	75,564	50,785
Other assets	11,254	17,154
TOTAL ASSETS	6,265,872	6,439,988
CURRENT LIABILITIES:
Accounts payable	143,732	135,628
Accrued liabilities	237,935	208,683
Current liabilities - discontinued operations	1	74
Total current liabilities	381,668	344,385
NONCURRENT LIABILITIES:
Long-term debt	493,700	492,902
Deferred income taxes	833,738	1,332,689
Other	99,727	101,409
Noncurrent liabilities - discontinued operations	14,548	4,012
Total noncurrent liabilities	1,441,713	1,931,012
SHAREHOLDERS' EQUITY:
Common stock	11,201	11,196
Additional paid-in capital	494,604	487,248
Retained earnings	4,103,418	3,855,686
Accumulated other comprehensive income	8,942	2,300
Treasury stock, at cost	(175,674)	(191,839)
Total shareholders' equity	4,442,491	4,164,591
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,265,872	6,439,988
Eliminations
Current assets:
Accounts receivable, net of allowance	(116)	(5)
Prepaid expenses and other	(818)	(442)
Total current assets	(934)	(447)
Intercompany	(2,480,143)	(2,086,087)
Investment in subsidiaries	(6,157,695)	(5,653,432)
TOTAL ASSETS	(8,638,772)	(7,739,966)
CURRENT LIABILITIES:
Accounts payable	(109)
Accrued liabilities	(826)	(447)
Total current liabilities	(935)	(447)
NONCURRENT LIABILITIES:
Intercompany	(2,480,043)	(2,085,987)
Total noncurrent liabilities	(2,480,043)	(2,085,987)
SHAREHOLDERS' EQUITY:
Common stock	(100)	(100)
Additional paid-in capital	(53,476)	(53,476)
Retained earnings	(6,091,134)	(5,593,218)
Accumulated other comprehensive income	(13,084)	(6,738)
Total shareholders' equity	(6,157,794)	(5,653,532)
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	(8,638,772)	(7,739,966)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Cash and cash equivalents	(1,027)	(587)	(1,141)	(955)
Accounts receivable, net of allowance	(164)	766
Prepaid expenses and other	15,707	12,200
Total current assets	14,516	12,379
Investments	14,902	13,853
Property, plant and equipment, net	48,273	49,851
Intercompany	150,255	90,885
Other assets	5,162	4,955
Investment in subsidiaries	5,970,971	5,470,050
TOTAL ASSETS	6,204,079	5,641,973
CURRENT LIABILITIES:
Accounts payable	82,019	82,360
Accrued liabilities	33,009	26,698
Total current liabilities	115,028	109,058
NONCURRENT LIABILITIES:
Deferred income taxes	(7,230)	(11,201)
Intercompany	1,627,211	1,354,068
Other	26,579	25,457
Total noncurrent liabilities	1,646,560	1,368,324
SHAREHOLDERS' EQUITY:
Common stock	11,201	11,196
Additional paid-in capital	494,604	487,248
Retained earnings	4,103,418	3,855,686
Accumulated other comprehensive income	8,942	2,300
Treasury stock, at cost	(175,674)	(191,839)
Total shareholders' equity	4,442,491	4,164,591
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,204,079	5,641,973
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	292,439	508,091	559,945	899,028
Short-term investments	44,279	44,491
Accounts receivable, net of allowance	482,746	411,599
Inventories	119,043	102,470
Prepaid expenses and other	13,353	6,383
Total current assets	951,860	1,073,034
Investments	77,800	70,173
Property, plant and equipment, net	4,520,974	4,609,144
Intercompany	2,079,427	1,746,662
Other assets	921	3,839
Investment in subsidiaries	186,724	183,382
TOTAL ASSETS	7,817,706	7,686,234
CURRENT LIABILITIES:
Accounts payable	55,242	48,679
Accrued liabilities	163,373	148,491
Total current liabilities	218,615	197,170
NONCURRENT LIABILITIES:
Long-term debt	493,700	492,902
Deferred income taxes	820,472	1,286,381
Intercompany	277,742	210,823
Other	50,569	43,471
Total noncurrent liabilities	1,642,483	2,033,577
SHAREHOLDERS' EQUITY:
Common stock	100	100
Additional paid-in capital	52,436	52,437
Retained earnings	5,890,988	5,396,212
Accumulated other comprehensive income	13,084	6,738
Total shareholders' equity	5,956,608	5,455,487
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	7,817,706	7,686,234
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	15,014	13,871	$ 13,983	$ 7,488
Accounts receivable, net of allowance	82,855	64,714
Inventories	33,066	34,734
Prepaid expenses and other	37,101	36,979
Current assets - discontinued operations		3
Total current assets	168,036	150,301
Property, plant and equipment, net	314,131	342,056
Intercompany	250,461	248,540
Goodwill	69,496	51,705
Intangible assets, net	75,564	50,785
Other assets	5,171	8,360
TOTAL ASSETS	882,859	851,747
CURRENT LIABILITIES:
Accounts payable	6,580	4,589
Accrued liabilities	42,379	33,941
Current liabilities - discontinued operations	1	74
Total current liabilities	48,960	38,604
NONCURRENT LIABILITIES:
Deferred income taxes	20,496	57,509
Intercompany	575,090	521,096
Other	22,579	32,481
Noncurrent liabilities - discontinued operations	14,548	4,012
Total noncurrent liabilities	632,713	615,098
SHAREHOLDERS' EQUITY:
Additional paid-in capital	1,040	1,039
Retained earnings	200,146	197,006
Total shareholders' equity	201,186	198,045
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 882,859	$ 851,747