Guarantor Subsidiaries (Consolidating Statement Of Cash Flow) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Activities
Net income (loss)	$ 47,600	$ 69,200	$ 32,700	$ 5,300	$ 77,500	$ 45,000	$ 51,800	$ 39,300	$ 154,800	$ 213,600	$ 245,300
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization									152,200	134,000	116,900
Deferred and noncurrent income tax charges (benefits)									23,700	37,900	(50,400)
Accretion of asset retirement obligations									2,100	2,000	1,800
Pretax (gains) losses from sale of assets									(100)	1,100	3,900
Net decrease (increase) in noncash operating working capital									(48,700)	2,300	(36,900)
Equity in earnings									0	0	0
Loss on early debt extinguishment									14,800	0	0
Other operating activities - net									14,500	7,800	3,000
Net cash provided by operating activities									313,300	398,700	283,600
Investing Activities
Property additions									(204,800)	(204,300)	(258,300)
Proceeds from sale of assets									2,500	1,200	900
Other investing activities - net									(800)	(6,000)	(4,700)
Net cash required by investing activities									(203,100)	(209,100)	(262,100)
Financing Activities
Purchase of treasury stock									(165,800)	(144,400)	(206,000)
Repayments of debt									(573,400)	(21,300)	(131,400)
Borrowings of debt									743,800	0	338,800
Early debt extinguishment costs									(10,400)	0	0
Debt issuance costs									(4,100)	0	(1,100)
Amounts related to share-based compensation									(4,500)	(9,400)	(5,600)
Net distributions to parent									0	0	0
Net cash required by financing activities									(14,400)	(175,100)	(5,300)
Net change in cash, cash equivalents, and restricted cash									95,800	14,500	16,200
Cash, cash equivalents, and restricted cash at January 1				184,500				170,000	184,500	170,000	153,800
Cash, cash equivalents, and restricted cash at December 31	280,300				184,500				280,300	184,500	170,000
Reconciliation of Cash, Cash Equivalents and Restricted Cash
Cash and cash equivalents												$ 280,300	$ 184,500	$ 170,000	$ 153,800
Restricted cash												0	0	0	0
Cash, cash equivalents, and restricted cash	280,300			184,500	184,500			170,000	280,300	170,000	170,000	280,300	184,500	170,000	153,800
Eliminations
Operating Activities
Net income (loss)									(154,300)	(213,100)	(245,300)
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization									0	0	0
Deferred and noncurrent income tax charges (benefits)									0	0	0
Accretion of asset retirement obligations									0	0	0
Pretax (gains) losses from sale of assets									0	0	0
Net decrease (increase) in noncash operating working capital									0	0	0
Equity in earnings									154,300	213,100	245,300
Loss on early debt extinguishment									0
Other operating activities - net									0	0	0
Net cash provided by operating activities									0	0	0
Investing Activities
Property additions									0	0	0
Proceeds from sale of assets									0	0	0
Other investing activities - net									0	0	0
Net cash required by investing activities									0	0	0
Financing Activities
Purchase of treasury stock									0	0	0
Repayments of debt									0	0	0
Borrowings of debt									0	0	0
Early debt extinguishment costs									0
Debt issuance costs									0	0	0
Amounts related to share-based compensation									0	0	0
Net distributions to parent									0	0	0
Net cash required by financing activities									0	0	0
Net change in cash, cash equivalents, and restricted cash									0	0	0
Cash, cash equivalents, and restricted cash at January 1				0				0	0	0	0
Cash, cash equivalents, and restricted cash at December 31	0				0				0	0	0
Reconciliation of Cash, Cash Equivalents and Restricted Cash
Cash and cash equivalents												0	0	0	0
Restricted cash												0	0	0	0
Cash, cash equivalents, and restricted cash	0			0	0			0	0	0	0	0	0	0	0
Parent Company
Financing Activities
Early debt extinguishment costs									0
Parent Company | Reportable Legal Entities
Operating Activities
Net income (loss)									320,600	1,187,300	245,300
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization									0	0	0
Deferred and noncurrent income tax charges (benefits)									0	0	0
Accretion of asset retirement obligations									0	0	0
Pretax (gains) losses from sale of assets									0	0	0
Net decrease (increase) in noncash operating working capital									0	0	0
Equity in earnings									(154,800)	(213,600)	(245,300)
Loss on early debt extinguishment									0
Other operating activities - net									0	0	0
Net cash provided by operating activities									165,800	973,700	0
Investing Activities
Property additions									0	0	0
Proceeds from sale of assets									0	0	0
Other investing activities - net									0	0	0
Net cash required by investing activities									0	0	0
Financing Activities
Purchase of treasury stock									(165,800)	(144,400)	(206,000)
Repayments of debt									0	0	0
Borrowings of debt									0	0	0
Debt issuance costs									0	0	0
Amounts related to share-based compensation									0	0	0
Net distributions to parent									0	(829,300)	206,000
Net cash required by financing activities									(165,800)	(973,700)	0
Net change in cash, cash equivalents, and restricted cash									0	0	0
Cash, cash equivalents, and restricted cash at January 1				0				0	0	0	0
Cash, cash equivalents, and restricted cash at December 31	0				0				0	0	0
Reconciliation of Cash, Cash Equivalents and Restricted Cash
Cash and cash equivalents												0	0	0	0
Restricted cash												0	0	0	0
Cash, cash equivalents, and restricted cash	0			0	0			0	0	0	0	0	0	0	0
Issuer
Financing Activities
Early debt extinguishment costs									(10,400)
Issuer | Reportable Legal Entities
Operating Activities
Net income (loss)									(11,000)	(760,100)	245,300
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization									152,100	134,000	116,900
Deferred and noncurrent income tax charges (benefits)									23,700	37,900	(50,400)
Accretion of asset retirement obligations									2,100	2,000	1,800
Pretax (gains) losses from sale of assets									(100)	1,100	3,900
Net decrease (increase) in noncash operating working capital									(48,600)	2,400	(36,900)
Equity in earnings									500	500	0
Loss on early debt extinguishment									14,800
Other operating activities - net									14,500	7,800	3,000
Net cash provided by operating activities									148,000	(574,400)	283,600
Investing Activities
Property additions									(198,800)	(203,100)	(257,100)
Proceeds from sale of assets									2,500	1,200	900
Other investing activities - net									(800)	(6,000)	(4,700)
Net cash required by investing activities									(197,100)	(207,900)	(260,900)
Financing Activities
Purchase of treasury stock									0	0	0
Repayments of debt									(573,400)	(21,300)	(131,400)
Borrowings of debt									743,800	0	338,800
Debt issuance costs									(4,100)	0	(1,100)
Amounts related to share-based compensation									(4,500)	(9,400)	(5,600)
Net distributions to parent									(6,900)	827,100	(207,300)
Net cash required by financing activities									144,500	796,400	(6,600)
Net change in cash, cash equivalents, and restricted cash									95,400	14,100	16,100
Cash, cash equivalents, and restricted cash at January 1				184,000				169,900	184,000	169,900	153,800
Cash, cash equivalents, and restricted cash at December 31	279,400				184,000				279,400	184,000	169,900
Reconciliation of Cash, Cash Equivalents and Restricted Cash
Cash and cash equivalents												279,400	184,000	169,900	153,800
Restricted cash												0	0	0	0
Cash, cash equivalents, and restricted cash	279,400			184,000	184,000			169,900	184,000	169,900	169,900	279,400	184,000	169,900	153,800
Guarantor Subsidiaries
Financing Activities
Early debt extinguishment costs									0
Guarantor Subsidiaries | Reportable Legal Entities
Operating Activities
Net income (loss)									(500)	(500)	0
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization									100	0	0
Deferred and noncurrent income tax charges (benefits)									0	0	0
Accretion of asset retirement obligations									0	0	0
Pretax (gains) losses from sale of assets									0	0	0
Net decrease (increase) in noncash operating working capital									(100)	(100)	0
Equity in earnings									0	0	0
Loss on early debt extinguishment									0
Other operating activities - net									0	0	0
Net cash provided by operating activities									(500)	(600)	0
Investing Activities
Property additions									(6,000)	(1,200)	(1,200)
Proceeds from sale of assets									0	0	0
Other investing activities - net									0	0	0
Net cash required by investing activities									(6,000)	(1,200)	(1,200)
Financing Activities
Purchase of treasury stock									0	0	0
Repayments of debt									0	0	0
Borrowings of debt									0	0	0
Debt issuance costs									0	0	0
Amounts related to share-based compensation									0	0	0
Net distributions to parent									6,900	2,200	1,300
Net cash required by financing activities									6,900	2,200	1,300
Net change in cash, cash equivalents, and restricted cash									400	400	100
Cash, cash equivalents, and restricted cash at January 1				500				100	500	100	0
Cash, cash equivalents, and restricted cash at December 31	900				500				900	500	100
Reconciliation of Cash, Cash Equivalents and Restricted Cash
Cash and cash equivalents												900	500	100	0
Restricted cash												0	0	0	0
Cash, cash equivalents, and restricted cash	900			500	500			100	900	100	0	900	500	100	0
Non-Guarantor Subsidiaries
Financing Activities
Early debt extinguishment costs									0
Non-Guarantor Subsidiaries | Reportable Legal Entities
Operating Activities
Net income (loss)									0	0	0
Adjustments to reconcile net income to net cash provided by operating activities
Depreciation and amortization									0	0	0
Deferred and noncurrent income tax charges (benefits)									0	0	0
Accretion of asset retirement obligations									0	0	0
Pretax (gains) losses from sale of assets									0	0	0
Net decrease (increase) in noncash operating working capital									0	0	0
Equity in earnings									0	0	0
Loss on early debt extinguishment									0
Other operating activities - net									0	0	0
Net cash provided by operating activities									0	0	0
Investing Activities
Property additions									0	0	0
Proceeds from sale of assets									0	0	0
Other investing activities - net									0	0	0
Net cash required by investing activities									0	0	0
Financing Activities
Purchase of treasury stock									0	0	0
Repayments of debt									0	0	0
Borrowings of debt									0	0	0
Debt issuance costs									0	0	0
Amounts related to share-based compensation									0	0	0
Net distributions to parent									0	0	0
Net cash required by financing activities									0	0	0
Net change in cash, cash equivalents, and restricted cash									0	0	0
Cash, cash equivalents, and restricted cash at January 1				0				0	0	0	0
Cash, cash equivalents, and restricted cash at December 31	0				0				0	0	0
Reconciliation of Cash, Cash Equivalents and Restricted Cash
Cash and cash equivalents												0	0	0	0
Restricted cash												0	0	0	0
Cash, cash equivalents, and restricted cash	$ 0			$ 0	$ 0			$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0