Supplemental Gas Data Results of Operations (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Jun. 30, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Extractive Industries [Abstract]
Natural Gas, NGLs and Oil Revenue			$ 1,577,937	$ 1,125,224	$ 793,248
(Loss) Gain on Commodity Derivative Instruments			(30,212)	206,930	(141,021)
Gas Royalty Interests and Purchased Gas Sales			65,986	53,795	43,256
Total Revenue			1,613,711	1,385,949	695,483
Lease Operating Expense			95,139	88,932	96,434
Production, Ad Valorem, and Other Fees			32,750	29,267	31,049
Transportation, Gathering and Compression			424,206	382,865	374,350
Purchased Gas Costs			64,817	52,597	42,717
Impairment of Exploration and Production Properties	$ 18,650	$ 137,865	0	137,865	0
Exploration Costs			12,033	48,074	14,522
DD&A			461,149	412,036	419,939
Total Costs			1,090,094	1,151,636	979,011
Pre-tax Operating Income / (Loss)			523,617	234,313	(283,528)
Income Tax Benefit			102,629	(348,676)	(69,929)
Results of Operations for Producing Activities excluding Corporate and Interest Costs			$ 420,988	$ 582,989	$ (213,599)