Impairment Expense (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018
Impairment of Oil and Gas Properties [Line Items]
Impairment of Oil and Gas Properties		$ 2,099,780		$ 517,896	$ 347,021
Proved Properties
Impairment of Oil and Gas Properties [Line Items]
Impairment of Oil and Gas Properties	[1]	1,268,073		206,469	120,859
Unproved Properties
Impairment of Oil and Gas Properties [Line Items]
Impairment of Oil and Gas Properties	[2]	472,143		220,444	173,383
Leasehold Costs of Impairments of Unproved Oil and Gas Properties		252,000
Other Assets
Impairment of Oil and Gas Properties [Line Items]
Other Assets Impairments Related to Commodity Price-Related Write-Down		72,000		90,000	49,000
Impairment of Oil and Gas Properties	[3]	299,851		90,983	48,732
Impairments for Owned and Leased Sand and Crude-by-Rail Assets		228,000
Inventories
Impairment of Oil and Gas Properties [Line Items]
Impairment of Oil and Gas Properties		0		0	4,047
Firm Commitments Contracts
Impairment of Oil and Gas Properties [Line Items]
Impairment of Oil and Gas Properties		59,713	[4]	$ 0	$ 0
Canadian Firm Commitment Contracts		$ 60,000

[1] Impairments to proved oil and gas properties in 2020 included legacy and non-core natural gas and crude oil and combo plays. Impairments to proved oil and gas properties in 2019 and 2018 included domestic legacy natural gas assets. See Notes 1 and 13.

[2] Unproved properties with acquisition costs that are not individually significant are aggregated, and the portion of such costs estimated to be nonproductive is amortized over the remaining lease term. Unproved properties with individually significant acquisition costs are reviewed individually for impairment. Impairments of unproved oil and gas properties included charges of $252 million in 2020 for certain leasehold costs that are no longer expected to be developed before expiration. See Note 1.

[3] Includes impairment charges for owned and leased sand and crude-by-rail assets of $228 million in 2020 (see Note 18) and a commodity price-related write-down of other assets of $72 million, $90 million and $49 million in 2020, 2019 and 2018, respectively (see Note 13).

[4] Includes impairment charges of $60 million in 2020 for firm commitment contracts related to its decision to exit the Horn River Basin in British Columbia, Canada.