Guarantor and Non-Guarantor Financial Information - Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash provided by operating activities	$ 308,196	$ 393,309
INVESTING ACTIVITIES:
Capital expenditures	(114,470)	(369,981)
Purchase of short-term investments	(6,918)
Proceeds from sales of short-term investments	4,600
Proceeds from asset sales	6,058	7,160
Net cash used in investing activities	(110,730)	(362,821)
FINANCING ACTIVITIES:
Dividends paid	(74,560)	(74,822)
Repurchase of common stock		(59,654)
Debt issuance costs	(32)
Exercise of stock options, net of tax withholding	(59)	(2,062)
Tax withholdings related to net share settlements of restricted stock	(3,617)	(4,248)
Tax benefit (expense) from share-based compensation	(352)	2,723
Net cash used in financing activities	(78,620)	(138,063)
Net increase (decrease) in cash and cash equivalents	118,846	(107,575)
Cash and cash equivalents, beginning of period	729,384	360,307
Cash and cash equivalents, end of period	848,230	252,732
Guarantor/Parent | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash provided by operating activities	4,381	8,699
INVESTING ACTIVITIES:
Capital expenditures	(9,085)	(3,491)
Intercompany transfers	9,085	3,491
FINANCING ACTIVITIES:
Dividends paid	(74,560)	(74,822)
Intercompany transfers	74,560	134,476
Repurchase of common stock		(59,654)
Exercise of stock options, net of tax withholding	(59)	(2,062)
Tax withholdings related to net share settlements of restricted stock	(3,617)	(4,248)
Tax benefit (expense) from share-based compensation	(572)	33
Net cash used in financing activities	(4,248)	(6,277)
Net increase (decrease) in cash and cash equivalents	133	2,422
Cash and cash equivalents, beginning of period	(838)	(2,050)
Cash and cash equivalents, end of period	(705)	372
Issuer Subsidiary | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash provided by operating activities	307,138	367,674
INVESTING ACTIVITIES:
Capital expenditures	(103,510)	(341,499)
Intercompany transfers	(9,085)	(3,491)
Purchase of short-term investments	(6,918)
Proceeds from sales of short-term investments	4,600
Proceeds from asset sales	5,241	6,285
Net cash used in investing activities	(109,672)	(338,705)
FINANCING ACTIVITIES:
Intercompany transfers	(74,560)	(134,476)
Debt issuance costs	(32)
Tax benefit (expense) from share-based compensation	195	2,667
Net cash used in financing activities	(74,397)	(131,809)
Net increase (decrease) in cash and cash equivalents	123,069	(102,840)
Cash and cash equivalents, beginning of period	693,273	329,655
Cash and cash equivalents, end of period	816,342	226,815
Non-Guarantor Subsidiaries | Reportable Legal Entities
CONDENSED CONSOLIDATING STATEMENTS OF CASH FLOWS
Net cash provided by operating activities	(3,323)	16,936
INVESTING ACTIVITIES:
Capital expenditures	(1,875)	(24,991)
Proceeds from asset sales	817	875
Net cash used in investing activities	(1,058)	(24,116)
FINANCING ACTIVITIES:
Tax benefit (expense) from share-based compensation	25	23
Net cash used in financing activities	25	23
Net increase (decrease) in cash and cash equivalents	(4,356)	(7,157)
Cash and cash equivalents, beginning of period	36,949	32,702
Cash and cash equivalents, end of period	$ 32,593	$ 25,545