Guarantor and Non-Guarantor Financial Information - Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	$ 70,668	$ 308,196
INVESTING ACTIVITIES:
Capital expenditures	(82,127)	(114,470)
Purchase of short-term investments	(15,025)	(6,918)
Proceeds from sales of short-term investments	13,900	4,600
Proceeds from asset sales	1,209	6,058
Net cash used in investing activities	(82,043)	(110,730)
FINANCING ACTIVITIES:
Debt issuance costs		(32)
Dividends paid	(76,176)	(74,560)
Exercise of stock options, net of tax withholding	9,827	(59)
Tax withholdings related to net share settlements of restricted stock	(5,647)	(3,617)
Excess tax benefit from stock-based compensation	3,703	(352)
Net cash used in financing activities	(68,293)	(78,620)
Net increase (decrease) in cash and cash equivalents	(79,668)	118,846
Cash and cash equivalents, beginning of period	905,561	729,384
Cash and cash equivalents, end of period	825,893	848,230
Guarantor/Parent | Reportable Legal Entities
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	5,900	4,381
INVESTING ACTIVITIES:
Capital expenditures	(607)	(9,085)
Intercompany transfers	607	9,085
FINANCING ACTIVITIES:
Intercompany transfers	76,176	74,560
Dividends paid	(76,176)	(74,560)
Exercise of stock options, net of tax withholding	9,827	(59)
Tax withholdings related to net share settlements of restricted stock	(5,647)	(3,617)
Excess tax benefit from stock-based compensation	(403)	(572)
Net cash used in financing activities	3,777	(4,248)
Net increase (decrease) in cash and cash equivalents	9,677	133
Cash and cash equivalents, beginning of period	(955)	(838)
Cash and cash equivalents, end of period	8,722	(705)
Issuer Subsidiary | Reportable Legal Entities
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	65,679	307,138
INVESTING ACTIVITIES:
Capital expenditures	(80,556)	(103,510)
Intercompany transfers	(607)	(9,085)
Purchase of short-term investments	(15,025)	(6,918)
Proceeds from sales of short-term investments	13,900	4,600
Proceeds from asset sales	1,102	5,241
Net cash used in investing activities	(81,186)	(109,672)
FINANCING ACTIVITIES:
Debt issuance costs		(32)
Intercompany transfers	(76,176)	(74,560)
Excess tax benefit from stock-based compensation	3,701	195
Net cash used in financing activities	(72,475)	(74,397)
Net increase (decrease) in cash and cash equivalents	(87,982)	123,069
Cash and cash equivalents, beginning of period	899,028	693,273
Cash and cash equivalents, end of period	811,046	816,342
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONSOLIDATED CONDENSED STATEMENTS OF CASH FLOWS
Net cash provided by (used in) operating activities	(911)	(3,323)
INVESTING ACTIVITIES:
Capital expenditures	(964)	(1,875)
Proceeds from asset sales	107	817
Net cash used in investing activities	(857)	(1,058)
FINANCING ACTIVITIES:
Excess tax benefit from stock-based compensation	405	25
Net cash used in financing activities	405	25
Net increase (decrease) in cash and cash equivalents	(1,363)	(4,356)
Cash and cash equivalents, beginning of period	7,488	36,949
Cash and cash equivalents, end of period	$ 6,125	$ 32,593