Guarantor and Non-Guarantor Financial Information - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2015
Current assets:
Cash and cash equivalents	$ 383,664	$ 521,375	$ 825,893	$ 905,561
Short-term investments	42,541	44,491
Accounts receivable, net of reserve	535,271	477,074
Inventories	139,066	137,204
Prepaid expenses and other	65,802	55,120
Current assets of discontinued operations	2	3
Total current assets	1,166,346	1,235,267
INVESTMENTS	83,943	84,026
Net property, plant and equipment, at cost	4,950,400	5,001,051
Goodwill	69,347	51,705
Intangible assets, net of amortization	78,636	50,785
Other assets	13,424	17,154
TOTAL ASSETS	6,362,096	6,439,988
CURRENT LIABILITIES:
Accounts payable	126,936	135,628
Accrued liabilities	228,732	208,683
Current liabilities of discontinued operations	94	74
Total current liabilities	355,762	344,385
NONCURRENT LIABILITIES:
Long-term debt	493,168	492,902
Deferred income taxes	830,494	1,332,689
Other	87,852	101,409
Noncurrent liabilities of discontinued operations	4,470	4,012
Total noncurrent liabilities	1,415,984	1,931,012
SHAREHOLDERS' EQUITY:
Common stock	11,201	11,196
Additional paid-in capital	479,914	487,248
Retained earnings	4,278,326	3,855,686
Accumulated other comprehensive income (loss), after tax	2,039	2,300
Treasury stock, at cost	(181,130)	(191,839)
Total shareholders' equity	4,590,350	4,164,591
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,362,096	6,439,988
Eliminations
Current assets:
Accounts receivable, net of reserve	(5)	(5)
Prepaid expenses and other	(253)	(442)
Total current assets	(258)	(447)
Intercompany	(2,301,040)	(2,086,087)
Investment in subsidiaries	(6,181,913)	(5,653,432)
TOTAL ASSETS	(8,483,211)	(7,739,966)
CURRENT LIABILITIES:
Accrued liabilities	(258)	(447)
Total current liabilities	(258)	(447)
NONCURRENT LIABILITIES:
Intercompany	(2,300,940)	(2,085,987)
Total noncurrent liabilities	(2,300,940)	(2,085,987)
SHAREHOLDERS' EQUITY:
Common stock	(100)	(100)
Additional paid-in capital	(53,476)	(53,476)
Retained earnings	(6,122,062)	(5,593,218)
Accumulated other comprehensive income (loss), after tax	(6,375)	(6,738)
Total shareholders' equity	(6,182,013)	(5,653,532)
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	(8,483,211)	(7,739,966)
Guarantor/Parent | Reportable Legal Entities
Current assets:
Cash and cash equivalents	(1,956)	(587)	8,722	(955)
Accounts receivable, net of reserve	2,601	766
Prepaid expenses and other	24,284	12,200
Total current assets	24,929	12,379
INVESTMENTS	14,614	13,853
Net property, plant and equipment, at cost	48,723	49,851
Intercompany	140,441	90,885
Other assets	5,058	4,955
Investment in subsidiaries	5,977,108	5,470,050
TOTAL ASSETS	6,210,873	5,641,973
CURRENT LIABILITIES:
Accounts payable	81,921	82,360
Accrued liabilities	27,437	26,698
Total current liabilities	109,358	109,058
NONCURRENT LIABILITIES:
Deferred income taxes	(6,291)	(11,201)
Intercompany	1,491,283	1,354,068
Other	26,173	25,457
Total noncurrent liabilities	1,511,165	1,368,324
SHAREHOLDERS' EQUITY:
Common stock	11,201	11,196
Additional paid-in capital	479,914	487,248
Retained earnings	4,278,326	3,855,686
Accumulated other comprehensive income (loss), after tax	2,039	2,300
Treasury stock, at cost	(181,130)	(191,839)
Total shareholders' equity	4,590,350	4,164,591
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	6,210,873	5,641,973
Issuer Subsidiary | Reportable Legal Entities
Current assets:
Cash and cash equivalents	376,084	508,091	811,046	899,028
Short-term investments	42,541	44,491
Accounts receivable, net of reserve	433,812	411,599
Inventories	104,497	102,470
Prepaid expenses and other	6,506	6,383
Total current assets	963,440	1,073,034
INVESTMENTS	69,329	70,173
Net property, plant and equipment, at cost	4,568,909	4,609,144
Intercompany	1,912,973	1,746,662
Other assets	1,090	3,839
Investment in subsidiaries	204,805	183,382
TOTAL ASSETS	7,720,546	7,686,234
CURRENT LIABILITIES:
Accounts payable	39,454	48,679
Accrued liabilities	155,555	148,491
Total current liabilities	195,009	197,170
NONCURRENT LIABILITIES:
Long-term debt	493,168	492,902
Deferred income taxes	794,862	1,286,381
Intercompany	235,150	210,823
Other	37,975	43,471
Total noncurrent liabilities	1,561,155	2,033,577
SHAREHOLDERS' EQUITY:
Common stock	100	100
Additional paid-in capital	52,436	52,437
Retained earnings	5,905,471	5,396,212
Accumulated other comprehensive income (loss), after tax	6,375	6,738
Total shareholders' equity	5,964,382	5,455,487
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	7,720,546	7,686,234
Non-Guarantor Subsidiaries | Reportable Legal Entities
Current assets:
Cash and cash equivalents	9,536	13,871	$ 6,125	$ 7,488
Accounts receivable, net of reserve	98,863	64,714
Inventories	34,569	34,734
Prepaid expenses and other	35,265	36,979
Current assets of discontinued operations	2	3
Total current assets	178,235	150,301
Net property, plant and equipment, at cost	332,768	342,056
Intercompany	247,626	248,540
Goodwill	69,347	51,705
Intangible assets, net of amortization	78,636	50,785
Other assets	7,276	8,360
TOTAL ASSETS	913,888	851,747
CURRENT LIABILITIES:
Accounts payable	5,561	4,589
Accrued liabilities	45,998	33,941
Current liabilities of discontinued operations	94	74
Total current liabilities	51,653	38,604
NONCURRENT LIABILITIES:
Deferred income taxes	41,923	57,509
Intercompany	574,507	521,096
Other	23,704	32,481
Noncurrent liabilities of discontinued operations	4,470	4,012
Total noncurrent liabilities	644,604	615,098
SHAREHOLDERS' EQUITY:
Additional paid-in capital	1,040	1,039
Retained earnings	216,591	197,006
Total shareholders' equity	217,631	198,045
TOTAL LIABILITIES AND SHAREHOLDERS' EQUITY	$ 913,888	$ 851,747