14. Long-Lived Asset Impairment

We review long-lived assets, including property, plant and equipment and identifiable intangibles that are being amortized, for impairment whenever events or changes in circumstances, including the removal of compressor units from our active fleet, indicate that the carrying amount of an asset may not be recoverable.

We periodically review the future deployment of our idle compression assets for units that are not of the type, configuration, condition, make or model that are cost efficient to maintain and operate. Based on these reviews, we determine that certain idle compressor units should be retired from the active fleet. The retirement of these units from the active fleet triggers a review of these assets for impairment and as a result of our review, we may record an asset impairment to reduce the book value of each unit to its estimated fair value. The fair value of each unit is estimated based on the expected net sale proceeds compared to other fleet units we recently sold, a review of other units recently offered for sale by third parties or the estimated component value of the equipment we plan to use.

In connection with our review of our idle compression assets, we evaluate for impairment idle units that were culled from our fleet in prior years and are available for sale. Based on that review, we may reduce the expected proceeds from disposition and record additional impairment to reduce the book value of each unit to its estimated fair value.

The following table presents the results of our impairment review as recorded in our contract operations segment (dollars in thousands):



	Year Ended December 31,
	2018		2017		2016
Idle compressor units retired from the active fleet	310		325		655
Horsepower of idle compressor units retired from the active fleet	115,000		100,000		262,000
Impairment recorded on idle compressor units retired from the active fleet	$	28,127	$	26,287	$	76,693
Additional impairment recorded on available-for-sale compressor units previously culled	$	—	$	—	$	10,742

In addition to the impairment discussed above, $2.9 million of property, plant and equipment was impaired during the year ended December 31, 2017 as the result of physical asset observations and other events that indicated the carrying values of the assets, which were comprised of approximately 7,000 horsepower of idle compressor units, were not recoverable and $0.8 million of leasehold improvements and furniture and fixtures that were impaired in connection with the relocation of our corporate office. See Note 16 (“Corporate Office Relocation”) for further details.