Supplemental Financial Information (Parent - Cash Flow Statement) (Details 3) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Adjustments to reconcile net income (loss) to cash provided by operating activities:
Depletion and depreciation									$ 125,121,000	$ 401,200,000	$ 292,951,000
Ceiling test and other impairments	$ 0	$ 0	$ 0	$ 0	$ 3,144,899,000	$ 0	$ 0	$ 0	0	3,144,899,000	0
Deferred and current non-cash income taxes									1,000	(990,000)	995,000
Unrealized loss (gain) on commodity derivatives									0	104,190,000	(130,066,000)
Deferred gain on sale of liquids gathering system									(10,553,000)	(10,553,000)	(10,553,000)
(Gain) on sale of property									0	0	(8,022,000)
Stock compensation									5,562,000	4,128,000	5,467,000
Other									6,870,000	9,217,000	4,569,000
Net changes in operating assets and liabilities:
Restricted cash									(3,456,000)	2,000	2,000
Accounts receivable									(19,635,000)	65,132,000	(43,116,000)
Prepaid expenses and other									(15,647,000)	(20,106,000)	(1,920,000)
Other non-current assets									(539,000)	21,112,000	284,000
Accounts payable									(63,924,000)	13,815,000	28,696,000
Accrued liabilities									133,144,000	1,655,000	(5,938,000)
Production taxes payable									(7,944,000)	(3,312,000)	15,115,000
Interest payable									57,117,000	(3,441,000)	14,233,000
Other long-term obligations									276,000	(5,770,000)	6,427,000
Current taxes payable									2,547,000	1,580,000	609,000
Net cash provided by operating activities									307,614,000	515,538,000	712,584,000
Investing Activities:
Acquisition costs									0	3,964,000	(891,075,000)
Oil and gas property expenditures									(269,314,000)	(494,025,000)	(599,913,000)
Gathering system expenditures									0	0	(6,842,000)
Proceeds from sale of property									0	0	27,944,000
Change in capital cost accrual									(8,134,000)	(25,380,000)	(125,577,000)
Inventory									(1,123,000)	3,235,000	175,000
Purchase of property, plant and equipment									(329,000)	(551,000)	(5,455,000)
Net cash used in investing activities									(278,900,000)	(512,757,000)	(1,600,743,000)
Financing activities:
Borrowings on long-term debt									369,000,000	1,165,000,000	1,095,000,000
Payments on long-term debt									0	(1,153,000,000)	(1,037,000,000)
Proceeds from issuance of Senior Notes									0	0	850,000,000
Deferred financing costs									0	6,000	(13,245,000)
Repurchased shares/net share settlements									(379,000)	(2,514,000)	(9,111,000)
Payment of contingent consideration									0	(17,049,000)	0
Proceeds from exercise of options									0	0	770,000
Net cash provided by (used in) financing activities									368,621,000	(7,557,000)	886,414,000
(Decrease)/increase in cash during the period									397,335,000	(4,776,000)	(1,745,000)
Cash and cash equivalents, beginning of period				4,143,000				8,919,000	4,143,000	8,919,000	10,664,000
Cash and cash equivalents, end of period	401,478,000				4,143,000				401,478,000	4,143,000	8,919,000
Cash paid for:
Interest									4,793,000	169,867,000	108,889,000
Income taxes									94,000	0	1,752,000
Non Cash Investing Activities Oil And Gas Properties									0	0	20,000,000
Parent [Member]
Net changes in operating assets and liabilities:
Net cash provided by operating activities									(21,309,000)	(101,277,000)	(35,818,000)
Investing Activities:
Investment in subsidiary									0	0	(850,000,000)
Dividends received									24,089,000	96,297,000	52,741,000
Net cash used in investing activities									24,089,000	96,297,000	(797,259,000)
Financing activities:
Proceeds from issuance of Senior Notes									0	0	850,000,000
Deferred financing costs									0	6,000	(13,245,000)
Repurchased shares/net share settlements									43,000	0	(6,471,000)
Shares re-issued from treasury									(337,000)	4,725,000	2,936,000
Net cash provided by (used in) financing activities									(294,000)	4,731,000	833,220,000
(Decrease)/increase in cash during the period									2,486,000	(249,000)	143,000
Cash and cash equivalents, beginning of period				$ 523,000				$ 772,000	523,000	772,000	629,000
Cash and cash equivalents, end of period	$ 3,009,000				$ 523,000				$ 3,009,000	$ 523,000	$ 772,000