5. Property and Equipment

Property and equipment consisted of the following at September 30, 2018 and December 31, 2017 (in thousands):

	September 30,			December 31,
	2018			2017
Equipment	$	8,366,513		$	8,066,404
Oil and natural gas properties		218,511			211,566
Buildings		186,716			185,475
Land		26,144			26,593
Total property and equipment		8,797,884			8,490,038
Less accumulated depreciation, depletion and impairment		(4,716,984	)		(4,235,308	)
Property and equipment, net	$	4,080,900		$	4,254,730

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. During the three months ended September 30, 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 have limited commercial opportunity. The three and nine months ended September 30, 2018 include a charge of $48.4 million related to this retirement. The nine months ended September 30, 2017 includes 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, and during the three months ended September 30, 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 the comparable period of 2017.

In addition, the Company evaluates the recoverability of its long-lived assets whenever events or changes in circumstances indicate that their carrying amounts may not be recoverable (a “triggering event”). Based on recent commodity prices, the Company’s results of operations for the three and nine month periods ended September 30, 2018 and management’s expectations of operating results in future periods, the Company concluded that no triggering event occurred during the nine months ended September 30, 2018 with respect to its contract drilling segment, its pressure pumping segment, its directional drilling segment or its other operations, except for oil and natural gas properties, which are discussed in the following paragraph. Management’s expectations of future operating results were based on the assumption that activity levels in all segments and its other operations will remain relatively stable or improve in response to relatively stable or increasing oil prices.

The Company reviews its proved oil and natural gas properties for impairment whenever a triggering event occurs, such as downward revisions in reserve estimates or decreases in expected future oil and natural gas prices. Proved properties are grouped by field, and undiscounted cash flow estimates are prepared based on the Company’s expectation of future pricing over the lives of the respective fields. These cash flow estimates are reviewed by an independent petroleum engineer. If the net book value of a field exceeds its undiscounted cash flow estimate, impairment expense is measured and recognized as the difference between net book value and fair value. Impairment expense related to proved and unproved oil and natural gas properties in the three months and nine months ended September 30, 2018 was not material.