Summary of Significant Accounting Policies - Narrative (Details)				3 Months Ended	12 Months Ended
		Jan. 01, 2019 USD ($)	Nov. 12, 2018	Jun. 30, 2016 USD ($)	Dec. 31, 2018 USD ($) $ / Mcfe dry_hole segment	Dec. 31, 2017 USD ($) $ / Mcfe dry_hole shares	Dec. 31, 2016 USD ($) $ / Mcfe	Mar. 31, 2018 USD ($)	Jan. 01, 2018 USD ($)
Property, Plant and Equipment [Line Items]
Number of segments | segment					1
Unrealized holding gain (loss)					$ (2,600,000)	$ 1,500,000
Contract with customer, asset, net					783,000,000
Oil and gas joint interest billing receivables					324,200,000	149,300,000
Losses for lower of cost or market adjustments					0	0	$ 0
Percentage of ownership after transaction			19.90%
Internal costs					130,000,000	114,600,000	115,400,000
Interest costs capitalized					$ 29,000,000	$ 20,500,000	$ 19,200,000
Overall average rate of depletion | $ / Mcfe					1.04	1.04	1.06
Impairment/loss on sale of long-lived assets					$ 2,709,976,000	$ 0	$ 0
Lease impairments and expirations					279,708,000	7,552,000	15,686,000
Unproved properties					$ 4,166,048,000	$ 5,016,299,000	1,698,826,000
Number of exploratory dry holes | dry_hole					0	1
Capitalized exploratory well costs					$ 0	$ 0
Impairment of goodwill					$ 530,811,000	0	0
Number of days used in calculation					30 days
Amortization of intangible assets					$ 41,367,000	$ 5,400,000	0
Amortization expense, year 1					41,400,000
Amortization expense, year 2					$ 35,900,000
Largest amount of benefit threshold, percentage (no greater than)					50.00%
Stock options and awards (in shares) | shares						346,528
Anti-dilutive securities (in shares) | shares						429,785
Net loss of defined benefit pension plans, amortized from accumulated OCI, net of tax, into net periodic benefit cost							9,400,000
Benefits paid				$ 5,400,000
Expense recognized related to defined contribution plan					$ 17,300,000	$ 16,600,000	16,000,000
Asset retirement obligations, period increase					34,600,000	143,600,000	66,200,000
Measurement adjustments					14,400,000	(14,300,000)
Capitalization of non-cash stock based compensation					4,300,000	9,000,000	16,600,000
Increase in capital contributions payable					(274,200,000)	(4,400,000)	(27,700,000)
Liabilities assumed						10,000,000	87,600,000
Change in accounting principle	[1]								$ 4,113,000
Restricted cash							75,000,000
Mountain Valley Pipeline
Property, Plant and Equipment [Line Items]
Increase in capital contributions payable					$ (176,600,000)	$ (94,300,000)	$ (11,500,000)
Minimum | Subsequent Event
Property, Plant and Equipment [Line Items]
Right-of-use asset obtained		$ 100,000,000
Maximum | Subsequent Event
Property, Plant and Equipment [Line Items]
Right-of-use asset obtained		$ 125,000,000
Accounting Standards Update 2016-01
Property, Plant and Equipment [Line Items]
Change in accounting principle								$ 4,100,000
Noncompete Agreements
Property, Plant and Equipment [Line Items]
Intangible asset useful life					3 years
Oil and Gas Receivable
Property, Plant and Equipment [Line Items]
Contract with customer, asset, net					$ 783,000,000
Huron and Permian Basin of Texas
Property, Plant and Equipment [Line Items]
Impairment/loss on sale of long-lived assets					$ 2,400,000,000

[1] Related to adoption of ASU No. 2016-01. See Note 1 for additional information.