5. Property and Equipment

Property and equipment consisted of the following at September 30, 2017 and December 31, 2016 (in thousands):

	September 30,			December 31,
	2017			2016
Equipment	$	7,882,765		$	6,809,129
Oil and natural gas properties		209,912			201,568
Buildings		186,452			97,029
Land		26,630			22,270
Total property and equipment		8,305,759			7,129,996
Less accumulated depreciation, depletion and impairment		(4,107,474	)		(3,721,033	)
Property and equipment, net	$	4,198,285		$	3,408,963

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 (such as drilling conventional, vertical wells versus drilling longer, horizontal wells using higher specification rigs). 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 the second quarter of 2017, the Company recorded an impairment 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 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, 2017 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, 2017 with respect to its contract drilling segment, its pressure pumping 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 both segments and its other operations will remain relatively stable in response to relatively stable 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 totaled $1.3 million in the third quarter of 2017 and $3.5 million for the nine months ended September 30, 2017 and is included in depreciation, depletion, amortization and impairment in the condensed consolidated statements of operations.