GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash (used in) provided by operating activities	$ 544,531	$ 361,631	$ 754,531
Cash flows from investing activities:
Capital expenditures	(466,584)	(397,567)	(257,169)
Purchase of short-term investments	(71,049)	(69,866)	(57,276)
Payment for acquisition of business, net of cash acquired	(47,886)	(70,416)
Proceeds from sale of short-term investments	68,776	69,449	58,381
Proceeds from asset sales	44,381	23,412	21,845
Net cash used in investing activities	(472,362)	(444,988)	(234,219)
Cash flows from financing activities:
Payments on long-term debt			(40,000)
Debt issuance costs			(1,111)
Dividends paid	(308,430)	(305,515)	(300,152)
Payments for employee taxes on net settlement of equity awards	(7,114)	(6,599)	(5,646)
Proceeds from stock option exercises	6,355	11,285	2,774
Net cash used in financing activities	(309,189)	(300,829)	(344,135)
Net increase (decrease) in cash and cash equivalents	(237,020)	(384,186)	176,177
Cash and cash equivalents, beginning of period	521,375	905,561	729,384
Cash and cash equivalents, end of period	284,355	521,375	905,561
Guarantor/Parent | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash (used in) provided by operating activities	759	(4,686)	2,863
Cash flows from investing activities:
Capital expenditures	(12,723)	(4,264)	(16,119)
Payment for acquisition of business, net of cash acquired	(47,886)	(70,416)
Intercompany transfers	60,609	74,680	16,119
Proceeds from asset sales			9
Net cash used in investing activities			9
Cash flows from financing activities:
Intercompany transfers	308,430	305,515	300,152
Dividends paid	(308,430)	(305,515)	(300,152)
Payments for employee taxes on net settlement of equity awards	(7,114)	(6,599)	(5,646)
Proceeds from stock option exercises	6,355	11,285	2,774
Net cash used in financing activities	(759)	4,686	(2,872)
Issuer Subsidiary | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash (used in) provided by operating activities	539,476	354,711	777,756
Cash flows from investing activities:
Capital expenditures	(443,743)	(387,392)	(235,078)
Purchase of short-term investments	(71,049)	(69,866)	(57,276)
Proceeds from sale of short-term investments	68,776	69,449	58,381
Intercompany transfers	(60,609)	(74,680)	(16,119)
Proceeds from asset sales	41,289	22,724	19,237
Net cash used in investing activities	(465,336)	(439,765)	(230,855)
Cash flows from financing activities:
Payments on long-term debt			(40,000)
Debt issuance costs			(1,111)
Intercompany transfers	(308,430)	(305,515)	(300,152)
Net cash used in financing activities	(308,430)	(305,515)	(341,263)
Net increase (decrease) in cash and cash equivalents	(234,290)	(390,569)	205,638
Cash and cash equivalents, beginning of period	507,504	898,073	692,435
Cash and cash equivalents, end of period	273,214	507,504	898,073
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash (used in) provided by operating activities	4,296	11,606	(26,088)
Cash flows from investing activities:
Capital expenditures	(10,118)	(5,911)	(5,972)
Proceeds from asset sales	3,092	688	2,599
Net cash used in investing activities	(7,026)	(5,223)	(3,373)
Cash flows from financing activities:
Net increase (decrease) in cash and cash equivalents	(2,730)	6,383	(29,461)
Cash and cash equivalents, beginning of period	13,871	7,488	36,949
Cash and cash equivalents, end of period	$ 11,141	$ 13,871	$ 7,488