Guarantor and Non-Guarantor Financial Information - Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	$ 72,215	$ 74,371
INVESTING ACTIVITIES:
Capital expenditures	(91,698)	(82,127)
Purchase of short-term investments	(16,183)	(15,025)
Payment for acquisition of business, net of cash acquired	(47,832)
Proceeds from sale of short-term investments	18,120	13,900
Proceeds from asset sales	8,749	1,209
Net cash used in investing activities	(128,844)	(82,043)
FINANCING ACTIVITIES:
Dividends paid	(76,503)	(76,176)
Proceeds from stock option exercises	892	10,253
Payments for employee taxes on net settlement of equity awards	(5,471)	(6,073)
Net cash used in financing activities	(81,082)	(71,996)
Net decrease in cash and cash equivalents	(137,711)	(79,668)
Cash and cash equivalents, beginning of period	521,375
Cash and cash equivalents, end of period	383,664	825,893
Guarantor/Parent | Reportable Legal Entities
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	3,210	5,497
INVESTING ACTIVITIES:
Capital expenditures	(2,189)	(607)
Payment for acquisition of business, net of cash acquired	(47,832)
Intercompany transfers	50,021	607
FINANCING ACTIVITIES:
Intercompany transfers	76,503	76,176
Dividends paid	(76,503)	(76,176)
Proceeds from stock option exercises	892	10,253
Payments for employee taxes on net settlement of equity awards	(5,471)	(6,073)
Net cash used in financing activities	(4,579)	4,180
Net decrease in cash and cash equivalents	(1,369)	9,677
Cash and cash equivalents, beginning of period	(587)
Cash and cash equivalents, end of period	(1,956)	8,722
Issuer Subsidiary | Reportable Legal Entities
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	70,962	69,380
INVESTING ACTIVITIES:
Capital expenditures	(86,404)	(80,556)
Purchase of short-term investments	(16,183)	(15,025)
Proceeds from sale of short-term investments	18,120	13,900
Intercompany transfers	(50,021)	(607)
Proceeds from asset sales	8,022	1,102
Net cash used in investing activities	(126,466)	(81,186)
FINANCING ACTIVITIES:
Intercompany transfers	(76,503)	(76,176)
Net cash used in financing activities	(76,503)	(76,176)
Net decrease in cash and cash equivalents	(132,007)	(87,982)
Cash and cash equivalents, beginning of period	508,091
Cash and cash equivalents, end of period	376,084	811,046
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	(1,957)	(506)
INVESTING ACTIVITIES:
Capital expenditures	(3,105)	(964)
Proceeds from asset sales	727	107
Net cash used in investing activities	(2,378)	(857)
FINANCING ACTIVITIES:
Net decrease in cash and cash equivalents	(4,335)	(1,363)
Cash and cash equivalents, beginning of period	13,871
Cash and cash equivalents, end of period	$ 9,536	$ 6,125