Asset Retirement Obligations:

12 Months Ended

Dec. 31, 2020

Asset Retirement Obligation Disclosure [Abstract]

ASSET RETIREMENT OBLIGATIONS

We have identified legal retirement obligations related to reclamation of mining sites in the Mining segment, removal of fuel tanks, transformers containing polychlorinated biphenyls, and an evaporation pond at our Electric Utilities, wind turbines at our Electric Utilities and Power Generation segments, retirement of gas pipelines at our Gas Utilities and removal of asbestos at our Electric and Gas Utilities. We periodically review and update estimated costs related to these AROs. The actual cost may vary from estimates because of regulatory requirements, changes in technology and increased costs of labor, materials and equipment.

The following tables present the details of AROs which are included on the accompanying Consolidated Balance Sheets in Other deferred credits and other liabilities (in thousands):


	December 31, 2019		Liabilities Incurred		Liabilities Settled		Accretion		Revisions to Prior Estimates		December 31, 2020
Electric Utilities (a)	$	9,329	$	1,217	$	—	$	407	$	—	$	10,953
Gas Utilities (b)	36,085		4,782		(132)		1,539		—		42,274
Power Generation	4,739		—		—		206		—		4,945
Mining (c)	14,052		—		(185)		617		(1,225)		13,259
Total	64,205		$	5,999	$	(317)	$	2,769	$	(1,225)	$	71,431


	December 31, 2018		Liabilities Incurred		Liabilities Settled		Accretion		Revisions to Prior Estimates		December 31, 2019
Electric Utilities (d)	$	6,258	$	—	$	—	$	385	$	2,686	$	9,329
Gas Utilities	34,627		—		—		1,458		—		36,085
Power Generation (a)	300		3,445		—		158		836		4,739
Mining (c)	15,615		—		(380)		740		(1,923)		14,052
Total	$	56,800	$	3,445	$	(380)	$	2,741	$	1,599	$	64,205

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(a) Liabilities incurred were related to new wind assets.

(b) Liabilities incurred were driven by an increase in gas pipeline miles; which increases our legal liability for retirement of gas pipelines, specifically to purge and cap these lines in accordance with Federal regulations.

(c) The Mining Revisions to Prior Estimates were primarily driven by changes in estimated costs associated with back-filling the pit with overburden removed during the mining process.

(d) The Electric Utilities Revisions to Prior Estimates was primarily driven by an increase in the estimated cost to decommission certain regulated wind farm assets.

We also have legally required AROs related to certain assets within our electric transmission and distribution systems. These retirement obligations are pursuant to an easement or franchise agreement and are only required if we discontinue our utility service under such easement or franchise agreement. Accordingly, it is not possible to estimate a time period when these obligations could be settled and therefore, a liability for the cost of these obligations cannot be measured at this time.