Guarantor and Non-Guarantor Financial Information - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Current assets:
Cash and cash equivalents	$ 825,893	$ 905,561	$ 848,230	$ 729,384
Short-term investments	45,263	44,148
Accounts receivable, net of reserve	326,771	375,169
Inventories	126,082	124,325
Prepaid expenses and other	75,926	78,067
Assets held for sale	45,297	45,352
Current assets of discontinued operations	51	64
Total current assets	1,445,283	1,572,686
Investments	105,177	84,955
Property, plant and equipment, net	5,102,679	5,144,733
Other assets	24,498	29,645
Total assets	6,677,637	6,832,019
Current liabilities:
Accounts payable	111,638	95,422
Accrued liabilities	174,920	234,639
Current liabilities of discontinued operations	70	59
Total current liabilities	286,628	330,120
Noncurrent liabilities:
Long-term debt	492,110	491,847
Deferred income taxes	1,332,269	1,342,456
Other	86,359	102,781
Noncurrent liabilities of discontinued operations	4,356	3,890
Total noncurrent liabilities	1,915,094	1,940,974
Shareholders' equity:
Common stock	11,188	11,140
Additional paid-in capital	463,859	448,452
Retained earnings	4,178,235	4,289,807
Accumulated other comprehensive income (loss)	12,574	(204)
Treasury stock, at cost	(189,941)	(188,270)
Total shareholders' equity	4,475,915	4,560,925
Total liabilities and shareholders' equity	6,677,637	6,832,019
Eliminations
Current assets:
Accounts receivable, net of reserve	(1,279)	(1,279)
Prepaid expenses and other	(285)	(21,239)
Total current assets	(1,564)	(22,518)
Intercompany	(1,772,495)	(1,676,409)
Investment in subsidiaries	(5,772,364)	(5,787,831)
Total assets	(7,546,423)	(7,486,758)
Current liabilities:
Accounts payable	(1,274)	(1,274)
Accrued liabilities	(291)	20,234
Total current liabilities	(1,565)	18,960
Noncurrent liabilities:
Intercompany	(1,772,395)	(1,717,787)
Total noncurrent liabilities	(1,772,395)	(1,717,787)
Shareholders' equity:
Common stock	(100)	(100)
Additional paid-in capital	(52,592)	(48,082)
Retained earnings	(5,702,005)	(5,734,655)
Accumulated other comprehensive income (loss)	(17,766)	(5,094)
Total shareholders' equity	(5,772,463)	(5,787,931)
Total liabilities and shareholders' equity	(7,546,423)	(7,486,758)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Cash and cash equivalents	8,722	(955)	(705)	(838)
Accounts receivable, net of reserve	3,392	2
Prepaid expenses and other	12,019	6,928
Total current assets	24,133	5,975
Investments	13,411	13,431
Property, plant and equipment, net	56,028	59,173
Intercompany	16,161	16,147
Other assets	234	233
Investment in subsidiaries	5,563,500	5,579,713
Total assets	5,673,467	5,674,672
Current liabilities:
Accounts payable	79,896	80,000
Accrued liabilities	19,660	1,822
Total current liabilities	99,556	81,822
Noncurrent liabilities:
Deferred income taxes	(4,081)	(5,930)
Intercompany	1,081,580	1,016,673
Other	20,497	21,182
Total noncurrent liabilities	1,097,996	1,031,925
Shareholders' equity:
Common stock	11,188	11,140
Additional paid-in capital	463,859	448,452
Retained earnings	4,178,235	4,289,807
Accumulated other comprehensive income (loss)	12,574	(204)
Treasury stock, at cost	(189,941)	(188,270)
Total shareholders' equity	4,475,915	4,560,925
Total liabilities and shareholders' equity	5,673,467	5,674,672
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	811,046	899,028	816,342	693,273
Short-term investments	45,263	44,148
Accounts receivable, net of reserve	285,285	325,325
Inventories	90,061	87,946
Prepaid expenses and other	10,422	20,625
Assets held for sale	18,452	18,471
Total current assets	1,260,529	1,395,543
Investments	91,766	71,524
Property, plant and equipment, net	4,691,273	4,716,736
Intercompany	1,495,780	1,399,323
Other assets	6,814	267
Investment in subsidiaries	208,864	208,118
Total assets	7,755,026	7,791,511
Current liabilities:
Accounts payable	26,861	10,868
Accrued liabilities	126,784	176,985
Total current liabilities	153,645	187,853
Noncurrent liabilities:
Long-term debt	492,110	491,847
Deferred income taxes	1,308,141	1,303,324
Intercompany	215,050	209,276
Other	39,335	36,379
Total noncurrent liabilities	2,054,636	2,040,826
Shareholders' equity:
Common stock	100	100
Additional paid-in capital	51,638	47,533
Retained earnings	5,477,241	5,510,105
Accumulated other comprehensive income (loss)	17,766	5,094
Total shareholders' equity	5,546,745	5,562,832
Total liabilities and shareholders' equity	7,755,026	7,791,511
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	6,125	7,488	$ 32,593	$ 36,949
Accounts receivable, net of reserve	39,373	51,121
Inventories	36,021	36,379
Prepaid expenses and other	53,770	71,753
Assets held for sale	26,845	26,881
Current assets of discontinued operations	51	64
Total current assets	162,185	193,686
Property, plant and equipment, net	355,378	368,824
Intercompany	260,554	260,939
Other assets	17,450	29,145
Total assets	795,567	852,594
Current liabilities:
Accounts payable	6,155	5,828
Accrued liabilities	28,767	35,598
Current liabilities of discontinued operations	70	59
Total current liabilities	34,992	41,485
Noncurrent liabilities:
Deferred income taxes	28,209	45,062
Intercompany	475,765	491,838
Other	26,527	45,220
Noncurrent liabilities of discontinued operations	4,356	3,890
Total noncurrent liabilities	534,857	586,010
Shareholders' equity:
Additional paid-in capital	954	549
Retained earnings	224,764	224,550
Total shareholders' equity	225,718	225,099
Total liabilities and shareholders' equity	$ 795,567	$ 852,594