Guarantor and Non-Guarantor Financial Information - Cash Flows (Details) - USD ($) $ in Thousands	9 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash (used in) provided by operating activities	$ 358,421	$ 240,255
INVESTING ACTIVITIES:
Capital expenditures	(322,658)	(300,275)
Purchase of short-term investments	(52,159)	(48,958)
Acquisition of business, net cash received	(47,886)	(70,416)
Proceeds from sale of short-term investments	52,470	53,150
Proceeds from asset sales	28,049	17,921
Net cash used in investing activities	(342,184)	(348,578)
FINANCING ACTIVITIES:
Dividends paid	(230,368)	(229,061)
Proceeds from stock option exercises	5,160	10,884
Payments for employee taxes on net settlement of equity awards	(5,978)	(6,274)
Net cash used in financing activities	(231,186)	(224,451)
Net decrease in cash and cash equivalents	(214,949)	(332,774)
Cash and cash equivalents, beginning of period	521,375	905,561
Cash and cash equivalents, end of period	306,426	572,787
Guarantor/Parent | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash (used in) provided by operating activities	378	(4,796)
INVESTING ACTIVITIES:
Capital expenditures	(8,725)	(2,344)
Acquisition of business, net cash received	(47,886)	(70,416)
Intercompany transfers	56,611	72,760
FINANCING ACTIVITIES:
Intercompany transfers	230,368	229,061
Dividends paid	(230,368)	(229,061)
Proceeds from stock option exercises	5,160	10,884
Payments for employee taxes on net settlement of equity awards	(5,978)	(6,274)
Net cash used in financing activities	(818)	4,610
Net decrease in cash and cash equivalents	(440)	(186)
Cash and cash equivalents, beginning of period	(587)	(955)
Cash and cash equivalents, end of period	(1,027)	(1,141)
Issuer Subsidiary | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash (used in) provided by operating activities	350,557	235,176
INVESTING ACTIVITIES:
Capital expenditures	(306,278)	(293,946)
Purchase of short-term investments	(52,159)	(48,958)
Proceeds from sale of short-term investments	52,470	53,150
Intercompany transfers	(56,611)	(72,760)
Proceeds from asset sales	26,737	17,316
Net cash used in investing activities	(335,841)	(345,198)
FINANCING ACTIVITIES:
Intercompany transfers	(230,368)	(229,061)
Net cash used in financing activities	(230,368)	(229,061)
Net decrease in cash and cash equivalents	(215,652)	(339,083)
Cash and cash equivalents, beginning of period	508,091	899,028
Cash and cash equivalents, end of period	292,439	559,945
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash (used in) provided by operating activities	7,486	9,875
INVESTING ACTIVITIES:
Capital expenditures	(7,655)	(3,985)
Proceeds from asset sales	1,312	605
Net cash used in investing activities	(6,343)	(3,380)
FINANCING ACTIVITIES:
Net decrease in cash and cash equivalents	1,143	6,495
Cash and cash equivalents, beginning of period	13,871	7,488
Cash and cash equivalents, end of period	$ 15,014	$ 13,983