Property and equipment consisted of the following at December 31, 2018 and 2017 (in thousands):

	2018			2017
Equipment	$	8,370,933		$	8,066,404
Oil and natural gas properties		219,855			211,566
Buildings		186,736			185,475
Land		26,144			26,593
Total property and equipment		8,803,668			8,490,038
Less accumulated depreciation, depletion and impairment		(4,801,119	)		(4,235,308	)
Property and equipment, net	$	4,002,549		$	4,254,730

Depreciation, depletion, amortization and impairment — The following table summarizes depreciation, depletion, amortization and impairment expense related to property and equipment and intangible assets and liabilities for 2018, 2017 and 2016 (in thousands):

	2018		2017		2016
Depreciation and impairment expense	$	887,155	$	753,510	$	657,571
Amortization expense		18,197		21,764		3,643
Depletion expense		10,966		8,067		7,220
Total	$	916,318	$	783,341	$	668,434

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 them 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 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. In 2016, the Company retired 19 mechanical rigs but recorded no charge as it had written down mechanical rigs that were still marketed in 2015.

The Company also periodically evaluates its pressure pumping assets, and in 2018, the Company recorded a charge of $17.4 million for the write-down of pressure pumping equipment. The pressure pumping equipment was primarily obsolete sand handling equipment, which has been replaced with more efficient sand solutions. There were no similar charges in 2017 or 2016.

The Company reviews its long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that their 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 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. 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 utilization, revenue and costs for the Company’s equipment and services that were estimated based upon the Company’s existing contract backlog, as well as recent contract tenders and customer inquiries. 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. While the Company believes these assumptions with respect to future oil pricing are reasonable, actual future prices may vary significantly from the ones that were assumed. The timeframe over which oil prices will recover is highly uncertain. Potential events that could affect the Company’s assumptions regarding future prices and the timeframe for a recovery are affected by factors such as those described in “Risk Factors–We Are Dependent on the Oil and Natural Gas Industry and Market Prices for Oil and Natural Gas. Declines in Customers’ Operating and Capital Expenditures and in Oil and Natural Gas Prices May Adversely Affect Our Operating Results.”

All of these factors are beyond the Company’s control. If the lower oil price environment experienced at the end of 2018 were to last into 2020 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 years ended December 31, 2017 or 2016 with respect to its asset groups based on the Company’s results of operations for the years ended December 31, 2017 and 2016, management’s expectations of operating results in future periods and the prevailing commodity prices at the time.