Guarantor and Non-Guarantor Financial Information - Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015
Current assets:
Cash and cash equivalents	$ 572,787	$ 905,561	$ 907,032	$ 729,384
Short-term investments	39,894	44,148
Accounts receivable, net of reserve	440,872	375,169
Inventories	138,403	124,325
Prepaid expenses and other	58,425	78,067
Assets held for sale		45,352
Current assets of discontinued operations	7	64
Total current assets	1,250,388	1,572,686
Investments	76,986	84,955
Property, plant and equipment, at cost	5,062,914	5,144,733
Goodwill	51,967	4,718
Intangible assets, net of amortization	51,569	919
Other assets	20,067	24,008
TOTAL ASSETS	6,513,891	6,832,019
CURRENT LIABILITIES:
Accounts payable	137,206	95,422
Accrued liabilities	196,643	234,639
Current liabilities of discontinued operations	80	59
Total current liabilities	333,929	330,120
NONCURRENT LIABILITIES:
Carrying value of long-term fixed-rate debt	492,637	491,847
Deferred income taxes	1,325,250	1,342,456
Other	108,946	102,781
Noncurrent liabilities of discontinued operations	3,225	3,890
Total noncurrent liabilities	1,930,058	1,940,974
SHAREHOLDERS' EQUITY:
Common stock	11,190	11,140
Additional paid-in capital	478,231	448,452
Retained earnings	3,954,705	4,289,807
Accumulated other comprehensive income (loss), after tax	(4,101)	(204)
Treasury stock, at cost	(190,121)	(188,270)
Total shareholders' equity	4,249,904	4,560,925
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,513,891	6,832,019
Eliminations
Current assets:
Accounts receivable, net of reserve	(5)	(1,279)
Prepaid expenses and other	(30,255)	(21,239)
Total current assets	(30,260)	(22,518)
Intercompany	(2,096,646)	(1,676,409)
Investment in subsidiaries	(5,678,550)	(5,787,831)
TOTAL ASSETS	(7,805,456)	(7,486,758)
CURRENT LIABILITIES:
Accounts payable		(1,274)
Accrued liabilities	(30,260)	20,234
Total current liabilities	(30,260)	18,960
NONCURRENT LIABILITIES:
Intercompany	(2,096,546)	(1,717,787)
Total noncurrent liabilities	(2,096,546)	(1,717,787)
SHAREHOLDERS' EQUITY:
Common stock	(100)	(100)
Additional paid-in capital	(52,960)	(48,082)
Retained earnings	(5,624,710)	(5,734,655)
Accumulated other comprehensive income (loss), after tax	(880)	(5,094)
Total shareholders' equity	(5,678,650)	(5,787,931)
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	(7,805,456)	(7,486,758)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Cash and cash equivalents	(1,141)	(955)	(920)	(838)
Accounts receivable, net of reserve	2,095	2
Prepaid expenses and other	14,701	6,928
Total current assets	15,655	5,975
Investments	13,606	13,431
Property, plant and equipment, at cost	51,118	59,173
Intercompany	90,685	16,147
Other assets	4,992	233
Investment in subsidiaries	5,480,782	5,579,713
TOTAL ASSETS	5,656,838	5,674,672
CURRENT LIABILITIES:
Accounts payable	80,333	80,000
Accrued liabilities	26,104	1,822
Total current liabilities	106,437	81,822
NONCURRENT LIABILITIES:
Deferred income taxes	(9,099)	(5,930)
Intercompany	1,284,237	1,016,673
Other	25,359	21,182
Total noncurrent liabilities	1,300,497	1,031,925
SHAREHOLDERS' EQUITY:
Common stock	11,190	11,140
Additional paid-in capital	478,231	448,452
Retained earnings	3,954,705	4,289,807
Accumulated other comprehensive income (loss), after tax	(4,101)	(204)
Treasury stock, at cost	(190,121)	(188,270)
Total shareholders' equity	4,249,904	4,560,925
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	5,656,838	5,674,672
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	559,945	899,028	892,476	693,273
Short-term investments	39,894	44,148
Accounts receivable, net of reserve	380,873	325,325
Inventories	103,066	87,946
Prepaid expenses and other	7,159	20,625
Assets held for sale		18,471
Total current assets	1,090,937	1,395,543
Investments	63,380	71,524
Property, plant and equipment, at cost	4,658,226	4,716,736
Intercompany	1,736,767	1,399,323
Other assets	5,577	267
Investment in subsidiaries	197,768	208,118
TOTAL ASSETS	7,752,655	7,791,511
CURRENT LIABILITIES:
Accounts payable	52,929	10,868
Accrued liabilities	172,480	176,985
Total current liabilities	225,409	187,853
NONCURRENT LIABILITIES:
Carrying value of long-term fixed-rate debt	492,637	491,847
Deferred income taxes	1,272,699	1,303,324
Intercompany	255,190	209,276
Other	41,849	36,379
Total noncurrent liabilities	2,062,375	2,040,826
SHAREHOLDERS' EQUITY:
Common stock	100	100
Additional paid-in capital	51,993	47,533
Retained earnings	5,411,898	5,510,105
Accumulated other comprehensive income (loss), after tax	880	5,094
Total shareholders' equity	5,464,871	5,562,832
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	7,752,655	7,791,511
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	13,983	7,488	$ 15,476	$ 36,949
Accounts receivable, net of reserve	57,909	51,121
Inventories	35,337	36,379
Prepaid expenses and other	66,820	71,753
Assets held for sale		26,881
Current assets of discontinued operations	7	64
Total current assets	174,056	193,686
Property, plant and equipment, at cost	353,570	368,824
Intercompany	269,194	260,939
Goodwill	51,967	4,718
Intangible assets, net of amortization	51,569	919
Other assets	9,498	23,508
TOTAL ASSETS	909,854	852,594
CURRENT LIABILITIES:
Accounts payable	3,944	5,828
Accrued liabilities	28,319	35,598
Current liabilities of discontinued operations	80	59
Total current liabilities	32,343	41,485
NONCURRENT LIABILITIES:
Deferred income taxes	61,650	45,062
Intercompany	557,119	491,838
Other	41,738	45,220
Noncurrent liabilities of discontinued operations	3,225	3,890
Total noncurrent liabilities	663,732	586,010
SHAREHOLDERS' EQUITY:
Additional paid-in capital	967	549
Retained earnings	212,812	224,550
Total shareholders' equity	213,779	225,099
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 909,854	$ 852,594