Impairment Expense (Notes)

12 Months Ended

Dec. 31, 2020

Impairment expense was as follows for the years ended December 31, 2020, 2019 and 2018 (in thousands):


	2020		2019		2018

Proved properties (1)	$	1,268,073	$	206,469	$	120,859
Unproved properties (2)	472,143		220,444		173,383
Other assets (3)	299,851		90,983		48,732
Inventories	—		—		4,047
Firm commitment contracts (4)	59,713		—		—
Total	$	2,099,780	$	517,896	$	347,021

(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.