Property and equipment consisted of the following at December 31, 2019 and 2018 (in thousands):

	2019			2018
Equipment	$	8,114,326		$	8,370,933
Oil and natural gas properties		226,646			219,855
Buildings		184,700			186,736
Land		25,747			26,144
Total property and equipment		8,551,419			8,803,668
Less accumulated depreciation, depletion and impairment		(5,244,742	)		(4,801,119	)
Property and equipment, net	$	3,306,677		$	4,002,549

Depreciation, depletion, amortization and impairment — The following table summarizes depreciation, depletion, amortization and impairment expense related to property and equipment, intangible assets and liabilities for 2019, 2018 and 2017 (in thousands):

	2019		2018		2017
Depreciation and impairment expense	$	974,206	$	887,155	$	753,510
Amortization expense		17,722		18,197		21,764
Depletion expense		11,945		10,966		8,067
Total	$	1,003,873	$	916,318	$	783,341

On a periodic basis, the Company evaluates its fleet of drilling rigs for marketability based on the condition of inactive rigs, expenditures that would be necessary to bring inactive rigs to working condition and the expected demand for drilling services by rig type. The components comprising rigs that will no longer be marketed are evaluated, and those components with continuing utility to the Company’s other marketed rigs are transferred to other rigs or to the Company’s yards to be used as spare equipment. The remaining components of these rigs are retired. In 2019, the Company identified 36 legacy non-APEX® rigs and related equipment that would be retired. Based on the strong customer preference across the industry for super-spec drilling rigs, the Company believes the 36 rigs that were retired have limited commercial opportunity. The Company recorded a $173 million charge related to this retirement. In 2018, the Company identified 42 legacy non-APEX® rigs and related equipment that would be retired. Based on the strong customer preference across the industry for super-spec drilling rigs, the Company believes the 42 rigs that were retired had limited commercial opportunity. The Company recorded a $48.4 million charge related to this retirement. In 2017, the Company recorded a charge of $29.0 million for the write-down of drilling equipment with no continuing utility as a result of the upgrade of certain rigs to super-spec capability.

The Company also periodically evaluates its pressure pumping assets for marketability based on the condition of inactive equipment, expenditures that would be necessary to bring the equipment to working condition and the expected demand. The components of equipment that will no longer be marketed are evaluated, and those components with continuing utility will be used as parts to support active equipment. The remaining components of this equipment are retired. In 2019, the Company recorded a charge of $20.5 million for the write-down of pressure pumping equipment compared to a $17.4 million write-down of pressure pumping equipment in 2018. There was no similar charge in 2017.

The Company also periodically evaluates its directional drilling assets. During 2019, the Company recorded a charge of $8.4 million for the write-down of directional drilling equipment. There were no similar charges in 2018 or 2017.

The Company reviews its long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying amounts of certain assets may not be recovered over their estimated remaining useful lives (“triggering events”). In connection with this review, assets are grouped at the lowest level at which identifiable cash flows are largely independent of other asset groupings. The Company estimates future cash flows over the life of the respective assets or asset groupings in its assessment of impairment. These estimates of cash flows are based on historical cyclical trends in the industry as well as the Company’s expectations regarding the continuation of these trends in the future. Provisions for asset impairment are charged against income when estimated future cash flows, on an undiscounted basis, are less than the asset’s net book value. Any provision for impairment is measured at fair value.

Due to the decline in the market price of the Company’s common stock and recent commodity prices, the Company’s results of operations for the quarter ended September 30, 2019 and management’s expectations of operating results in future periods, the Company lowered its expectations with respect to future activity levels in certain of its operating segments. The Company deemed it necessary to assess the recoverability of its contract drilling, pressure pumping, directional drilling and oilfield rentals asset groups as of September 30, 2019. The Company performed an analysis as required by ASC 360-10-35 to assess the recoverability of the asset groups within its contract drilling, pressure pumping, directional drilling and oilfield rentals operating segments as of September 30, 2019. With respect to these asset groups, future cash flows were estimated over the expected remaining life of the assets, and the Company determined that, on an undiscounted basis, expected cash flows exceeded the carrying value of the asset groups, and no impairment was indicated. Expected cash flows, on an undiscounted basis, exceeded the carrying values of the asset groups within the contract drilling, pressure pumping, directional drilling and oilfield rentals operating segments by approximately 35%, 54%, 23% and 7%, respectively.

For the assessment performed in 2019, the expected cash flows for the Company’s asset groups included revenue growth rates, operating expense growth rates, and terminal growth rates. Also, the expected cash flows for the contract drilling, pressure pumping, directional drilling and oilfield rentals asset groups were based on the assumption that activity levels in all four segments would generally be lower than levels experienced in 2019 and would begin to recover in late 2020 or 2021 in response to improved oil prices. While the Company believes these assumptions with respect to future oil pricing are reasonable, actual future prices and activity levels may vary significantly from the ones that were assumed. The timeframe over which oil prices and activity levels may recover is highly uncertain.

All of these factors are beyond the Company’s control. If the lower oil price environment experienced in 2019 were to last into late 2021 and beyond, the Company’s actual cash flows would likely be less than the expected cash flows used in these assessments and could result in impairment charges in the future, and such impairment could be material.

The Company concluded that no triggering events occurred during the quarter ended December 31, 2019 with respect to its asset groups based on the Company’s results of operations for the current year, management’s expectations of operating results in future periods and the prevailing commodity prices at the time.

Due to the decline in the market price of the Company’s common stock and the deterioration of crude oil prices in the fourth quarter of 2018, the Company lowered its expectations with respect to future activity levels in certain of its operating segments. The Company deemed it necessary to assess the recoverability of its contract drilling, pressure pumping, directional drilling and oilfield rentals asset groups. The Company performed an analysis as required by ASC 360-10-35 to assess the recoverability of the asset groups within its contract drilling, pressure pumping, directional drilling and oilfield rentals operating segments as of December 31, 2018. With respect to these asset groups, future cash flows were estimated over the expected remaining life of the assets, and the Company determined that, on an undiscounted basis, expected cash flows exceeded the carrying value of the asset groups, and no impairment was indicated. Expected cash flows, on an undiscounted basis, exceeded the carrying values of the asset groups within the contract drilling, pressure pumping, directional drilling and oilfield rentals operating segments by approximately 38%, 58%, 9% and 23%, respectively.

For the assessment performed in 2018, the expected cash flows for the Company’s asset groups included revenue growth rates, operating expense growth rates, and terminal growth rates. Also, the expected cash flows for the contract drilling, pressure pumping, directional drilling and oilfield rentals asset groups were based on the assumption that activity levels in all four segments would generally be lower than levels experienced in the fourth quarter of 2018, and would begin to recover in late 2019 and continue into 2020 in response to improved oil prices and activity levels.

The Company concluded that no triggering events occurred during the year ended December 31, 2017 with respect to its asset groups based on the Company’s results of operations for the year ended December 31, 2017 management’s expectations of operating results in future periods and the prevailing commodity prices at the time.