GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	$ 357,217	$ 753,597	$ 1,428,574
INVESTING ACTIVITIES:
Capital expenditures	(397,567)	(257,169)	(1,131,445)
Purchase of short-term investments	(69,866)	(57,276)	(45,607)
Payment for acquisition of business, net of cash acquired	(70,416)
Proceeds from sale of short-term investments	69,449	58,381
Proceeds from asset sales	23,412	21,845	22,643
Net cash used in investing activities	(444,988)	(234,219)	(1,154,409)
FINANCING ACTIVITIES:
Payments on long-term debt		(40,000)	(40,000)
Proceeds from senior notes, net of discount			497,125
Debt issuance costs		(1,111)	(5,474)
Proceeds on short-term debt			1,002
Payments on short-term debt			(1,002)
Repurchase of common stock			(59,654)
Dividends paid	(305,515)	(300,152)	(298,367)
Exercise of stock options, net of tax withholding	10,534	1,040	2,650
Tax withholdings related to net share settlements of restricted stock	(5,848)	(3,912)	(5,140)
Excess tax benefit from stock-based compensation	4,414	934	3,772
Net cash provided by (used in) financing activities	(296,415)	(343,201)	94,912
Net increase (decrease) in cash and cash equivalents	(384,186)	176,177	369,077
Cash and cash equivalents, beginning of period	905,561	729,384	360,307
Cash and cash equivalents, end of period	521,375	905,561	729,384
Guarantor/Parent | Reportable Legal Entities
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	(3,828)	3,521	3,623
INVESTING ACTIVITIES:
Capital expenditures	(4,264)	(16,119)	(24,818)
Payment for acquisition of business, net of cash acquired	(70,416)
Intercompany transfers	74,680	16,119	24,818
Proceeds from asset sales		9	1
Net cash used in investing activities		9	1
FINANCING ACTIVITIES:
Repurchase of common stock			(59,654)
Intercompany transfers	305,515	300,152	358,021
Dividends paid	(305,515)	(300,152)	(298,367)
Exercise of stock options, net of tax withholding	10,534	1,040	2,650
Tax withholdings related to net share settlements of restricted stock	(5,848)	(3,912)	(5,140)
Excess tax benefit from stock-based compensation	(490)	(775)	78
Net cash provided by (used in) financing activities	4,196	(3,647)	(2,412)
Net increase (decrease) in cash and cash equivalents	368	(117)	1,212
Cash and cash equivalents, beginning of period	(955)	(838)	(2,050)
Cash and cash equivalents, end of period	(587)	(955)	(838)
Issuer Subsidiary | Reportable Legal Entities
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	349,929	776,364	1,379,707
INVESTING ACTIVITIES:
Capital expenditures	(387,392)	(235,078)	(1,064,288)
Purchase of short-term investments	(69,866)	(57,276)	(45,607)
Proceeds from sale of short-term investments	69,449	58,381
Intercompany transfers	(74,680)	(16,119)	(24,818)
Proceeds from asset sales	22,724	19,237	21,329
Net cash used in investing activities	(439,765)	(230,855)	(1,113,384)
FINANCING ACTIVITIES:
Payments on long-term debt		(40,000)	(40,000)
Proceeds from senior notes, net of discount			497,125
Debt issuance costs		(1,111)	(5,474)
Intercompany transfers	(305,515)	(300,152)	(358,021)
Excess tax benefit from stock-based compensation	4,414	1,509	3,665
Net cash provided by (used in) financing activities	(301,101)	(339,754)	97,295
Net increase (decrease) in cash and cash equivalents	(390,937)	205,755	363,618
Cash and cash equivalents, beginning of period	899,028	693,273	329,655
Cash and cash equivalents, end of period	508,091	899,028	693,273
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	11,116	(26,288)	45,244
INVESTING ACTIVITIES:
Capital expenditures	(5,911)	(5,972)	(42,339)
Proceeds from asset sales	688	2,599	1,313
Net cash used in investing activities	(5,223)	(3,373)	(41,026)
FINANCING ACTIVITIES:
Proceeds on short-term debt			1,002
Payments on short-term debt			(1,002)
Excess tax benefit from stock-based compensation	490	200	29
Net cash provided by (used in) financing activities	490	200	29
Net increase (decrease) in cash and cash equivalents	6,383	(29,461)	4,247
Cash and cash equivalents, beginning of period	7,488	36,949	32,702
Cash and cash equivalents, end of period	$ 13,871	$ 7,488	$ 36,949