Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Cash flows from operating activities:
Net income (loss)	$ 482,672	$ (128,212)	$ (56,828)
Adjustment for income from discontinued operations	10,338	349	3,838
Income (loss) from continuing operations	493,010	(127,863)	(52,990)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	583,802	585,543	598,587
Asset impairment charge	23,128		6,250
Amortization of debt discount and debt issuance costs	1,067	1,055	1,168
Provision for bad debt	2,193	2,016
(Recovery of) bad debt			(2,013)
Stock-based compensation	31,687	26,183	24,383
Pension settlement charge	913	1,640	4,964
(Gain) loss on investment securities	(1)		25,989
Gain from sale of assets	(22,660)	(20,627)	(9,896)
Deferred income tax (benefit) expense	(486,758)	(24,111)	60,088
Other	6,710	543	151
Change in assets and liabilities increasing (decreasing) cash:
Accounts receivable	(85,202)	(97,114)	72,792
Inventories of materials and supplies	(22,427)	(10,607)	1,944
Prepaid expenses and other	(955)	31,434	(2,460)
Accounts payable	(4,461)	39,412	(10,907)
Accrued liabilities	33,173	(36,120)	49,562
Deferred income tax liability	2,268	3,472	3,703
Other noncurrent liabilities	(10,787)	(13,075)	(16,831)
Net cash provided by operating activities from continuing operations	544,700	361,781	754,484
Net cash provided by (used in) operating activities from discontinued operations	(169)	(150)	47
Net cash 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)
Proceeds from stock option exercises	6,355	11,285	2,774
Payments for employee taxes on net settlement of equity awards	(7,114)	(6,599)	(5,646)
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
Cash paid during the period:
Interest paid	20,502	22,936	28,011
Income tax refund, net	38,400	23,463	24,109
Changes in accounts payable and accrued liabilities related to purchases of property, plant and equipment	$ (2,245)	$ (10,539)	$ 15,879