Significant Accounting Policies (Details) - USD ($)		1 Months Ended	3 Months Ended			12 Months Ended
	Jan. 03, 2018	May 31, 2018	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2019
Property, Plant and Equipment [Line Items]
Impairment of long-lived assets							$ 0	$ 355,681,000
Impairment of proved and unproved properties				$ 18,650,000	$ 137,865,000	$ 0	137,865,000	0
Goodwill acquired during period						796,359,000
Impairment of intangible assets						$ 18,650,000	$ 0	$ 0
Useful life, customer relationship intangible assets						17 years
Company's matching contribution, percentage						6.00%	6.00%	6.00%
Discretionary contributions to the Plan, amount						$ 0	$ 0	$ 2,761,000
Total payment and cost to the Plan						3,205,000	2,866,000	5,858,000
Reclassification of stranded tax effects						$ 0
Minimum
Property, Plant and Equipment [Line Items]
Discretionary contributions to the Plan, percentage						1.00%
Maximum
Property, Plant and Equipment [Line Items]
Discretionary contributions to the Plan, percentage						6.00%
Proved Properties
Property, Plant and Equipment [Line Items]
Impairment of proved and unproved properties						$ 0	0	0
Unproved Properties
Property, Plant and Equipment [Line Items]
Impairment of proved and unproved properties						$ 0	0	0
Building and Improvements | Minimum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment						10 years
Building and Improvements | Maximum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment						45 years
Machinery and Equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment						3 years
Machinery and Equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment						25 years
Gathering and Transmission | Minimum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment						30 years
Gathering and Transmission | Maximum
Property, Plant and Equipment [Line Items]
Useful life of property, plant and equipment						40 years
Knox and Coalfield
Property, Plant and Equipment [Line Items]
Impairment of long-lived assets							137,865,000
Exploration and Production Related Other Costs
Property, Plant and Equipment [Line Items]
Exploration expense						$ 12,033,000	$ 48,074,000	$ 14,522,000
Midstream
Property, Plant and Equipment [Line Items]
Goodwill acquired during period	$ 796,359,000
Other intangible assets	$ 128,781,000
Customer Relationships | Midstream
Property, Plant and Equipment [Line Items]
Impairment of intangible assets		$ 18,650,000				18,650,000
Useful life, customer relationship intangible assets		17 years
Accumulated Other Comprehensive Income (Loss)
Property, Plant and Equipment [Line Items]
Reclassification of stranded tax effects						(1,100,000)
Accumulated Other Comprehensive Income (Loss) | Accounting Standards Update 2018-02
Property, Plant and Equipment [Line Items]
Reclassification of stranded tax effects			$ (1,100,000)
Retained Earnings
Property, Plant and Equipment [Line Items]
Reclassification of stranded tax effects						$ 1,100,000
Retained Earnings | Accounting Standards Update 2018-02
Property, Plant and Equipment [Line Items]
Reclassification of stranded tax effects			$ 1,100,000
Subsequent Event | Accounting Standards Update 2016-02 | Minimum
Property, Plant and Equipment [Line Items]
Right-of-use asset									$ 200,000,000
Lease liability									210,000,000
Subsequent Event | Accounting Standards Update 2016-02 | Maximum
Property, Plant and Equipment [Line Items]
Right-of-use asset									225,000,000
Lease liability									$ 260,000,000