Guarantor and Non-Guarantor Financial Information - Balance Sheet (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Sep. 30, 2016	Mar. 31, 2016	Sep. 30, 2015
Current assets:
Cash and cash equivalents	$ 741,746	$ 905,561	$ 898,013	$ 729,384
Short-term investments	48,012	44,148
Accounts receivable, net of reserve	345,546	375,169
Inventories	131,679	124,325
Prepaid expenses and other	70,067	78,067
Assets held for sale	26,801	45,352
Current assets of discontinued operations	36	64
Total current assets	1,363,887	1,572,686
INVESTMENTS	88,299	84,955
PROPERTY, PLANT AND EQUIPMENT, at cost	5,061,368	5,144,733
Other assets	24,630	29,645
TOTAL ASSETS	6,538,184	6,832,019
CURRENT LIABILITIES:
Accounts payable	127,116	95,422
Accrued liabilities	174,261	234,639
Current liabilities of discontinued operations	40	59
Total current liabilities	301,417	330,120
NONCURRENT LIABILITIES:
Carrying value of long-term fixed-rate debt	492,373	491,847
Deferred income taxes	1,308,794	1,342,456
Other	83,552	102,781
Noncurrent liabilities of discontinued operations	4,654	3,890
Total noncurrent liabilities	1,889,373	1,940,974
SHAREHOLDERS' EQUITY:
Common stock	11,188	11,140
Additional paid-in capital	470,918	448,452
Retained earnings	4,052,974	4,289,807
Accumulated other comprehensive income (loss), after tax	2,433	(204)
Treasury stock, at cost	(190,119)	(188,270)
Total shareholders' equity	4,347,394	4,560,925
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,538,184	6,832,019
Eliminations
Current assets:
Accounts receivable, net of reserve	(986)	(1,279)
Prepaid expenses and other	(3,091)	(21,239)
Total current assets	(4,077)	(22,518)
Intercompany	(1,861,062)	(1,676,409)
Investment in subsidiaries	(5,704,566)	(5,787,831)
TOTAL ASSETS	(7,569,705)	(7,486,758)
CURRENT LIABILITIES:
Accounts payable	(981)	(1,274)
Accrued liabilities	(3,096)	20,234
Total current liabilities	(4,077)	18,960
NONCURRENT LIABILITIES:
Intercompany	(1,860,962)	(1,717,787)
Total noncurrent liabilities	(1,860,962)	(1,717,787)
SHAREHOLDERS' EQUITY:
Common stock	(100)	(100)
Additional paid-in capital	(52,778)	(48,082)
Retained earnings	(5,644,269)	(5,734,655)
Accumulated other comprehensive income (loss), after tax	(7,519)	(5,094)
Total shareholders' equity	(5,704,666)	(5,787,931)
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	(7,569,705)	(7,486,758)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Cash and cash equivalents	(2,681)	(955)	(1,900)	(838)
Accounts receivable, net of reserve	2,875	2
Prepaid expenses and other	10,274	6,928
Total current assets	10,468	5,975
INVESTMENTS	13,668	13,431
PROPERTY, PLANT AND EQUIPMENT, at cost	53,472	59,173
Intercompany	16,284	16,147
Other assets	234	233
Investment in subsidiaries	5,506,749	5,579,713
TOTAL ASSETS	5,600,875	5,674,672
CURRENT LIABILITIES:
Accounts payable	79,483	80,000
Accrued liabilities	13,655	1,822
Total current liabilities	93,138	81,822
NONCURRENT LIABILITIES:
Deferred income taxes	(5,472)	(5,930)
Intercompany	1,145,078	1,016,673
Other	20,737	21,182
Total noncurrent liabilities	1,160,343	1,031,925
SHAREHOLDERS' EQUITY:
Common stock	11,188	11,140
Additional paid-in capital	470,918	448,452
Retained earnings	4,052,974	4,289,807
Accumulated other comprehensive income (loss), after tax	2,433	(204)
Treasury stock, at cost	(190,119)	(188,270)
Total shareholders' equity	4,347,394	4,560,925
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	5,600,875	5,674,672
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	730,220	899,028	885,570	693,273
Short-term investments	48,012	44,148
Accounts receivable, net of reserve	316,580	325,325
Inventories	96,565	87,946
Prepaid expenses and other	2,647	20,625
Assets held for sale		18,471
Total current assets	1,194,024	1,395,543
INVESTMENTS	74,631	71,524
PROPERTY, PLANT AND EQUIPMENT, at cost	4,666,972	4,716,736
Intercompany	1,577,024	1,399,323
Other assets	8,101	267
Investment in subsidiaries	197,817	208,118
TOTAL ASSETS	7,718,569	7,791,511
CURRENT LIABILITIES:
Accounts payable	44,242	10,868
Accrued liabilities	137,964	176,985
Total current liabilities	182,206	187,853
NONCURRENT LIABILITIES:
Carrying value of long-term fixed-rate debt	492,373	491,847
Deferred income taxes	1,284,083	1,303,324
Intercompany	232,586	209,276
Other	36,036	36,379
Total noncurrent liabilities	2,045,078	2,040,826
SHAREHOLDERS' EQUITY:
Common stock	100	100
Additional paid-in capital	51,811	47,533
Retained earnings	5,431,855	5,510,105
Accumulated other comprehensive income (loss), after tax	7,519	5,094
Total shareholders' equity	5,491,285	5,562,832
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	7,718,569	7,791,511
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	14,207	7,488	$ 14,343	$ 36,949
Accounts receivable, net of reserve	27,077	51,121
Inventories	35,114	36,379
Prepaid expenses and other	60,237	71,753
Assets held for sale	26,801	26,881
Current assets of discontinued operations	36	64
Total current assets	163,472	193,686
PROPERTY, PLANT AND EQUIPMENT, at cost	340,924	368,824
Intercompany	267,754	260,939
Other assets	16,295	29,145
TOTAL ASSETS	788,445	852,594
CURRENT LIABILITIES:
Accounts payable	4,372	5,828
Accrued liabilities	25,738	35,598
Current liabilities of discontinued operations	40	59
Total current liabilities	30,150	41,485
NONCURRENT LIABILITIES:
Deferred income taxes	30,183	45,062
Intercompany	483,298	491,838
Other	26,779	45,220
Noncurrent liabilities of discontinued operations	4,654	3,890
Total noncurrent liabilities	544,914	586,010
SHAREHOLDERS' EQUITY:
Additional paid-in capital	967	549
Retained earnings	212,414	224,550
Total shareholders' equity	213,381	225,099
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 788,445	$ 852,594