Acquisitions and Divestitures (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Discontinued Operations and Disposal Groups [Abstract]
Net Decrease in Oil and Gas Properties From Purchase Price Allocation			$ (35,000)
Net Decrease in Deferred Tax Liabilities Noncurrent From Purchase Price Allocation			(32,000)
Cash and Cash Equivalents	$ 70,411		70,411
Accounts Receivable, Net	77,073		77,073
Inventories	10,955		10,955
Other	10,640		10,640
Total	169,079		169,079
Oil and Gas Properties (Successful Efforts Method)	3,815,207		3,815,207
Other Property, Plant and Equipment	21,824		21,824
Total Property, Plant and Equipment, Net	3,837,031		3,837,031
Other Assets	22,706		22,706
Total Assets	4,028,816		4,028,816
Accounts Payable	124,145		124,145
Accrued Taxes Payable	22,417		22,417
Other	743		743
Total	147,305		147,305
Long-Term Debt	163,829		163,829
Asset Retirement Obligations	163,144		163,144
Off-Market Transportation Contracts	39,720		39,720
Other Liabilities	28,645		28,645
Deferred Income Taxes	1,072,405		1,072,405
Total Liabilities	1,615,048		1,615,048
Total Consideration Transferred	2,413,768		2,413,768
Gain (Loss) on Disposition of Assets	$ (8,202)	$ 108,204	(33,876)	$ 101,801
Proceeds from Sale of Oil and Gas Property and Equipment			$ 191,593	$ 457,665