	(in thousands)
December 31,	2019			2018
Land and improvements, net	$	719,736		$	654,786
Buildings and improvements		1,413,690			1,283,182
Machinery and equipment		11,630,179			11,101,840
Proved oil and gas properties		558,123			557,383
Leasehold interest in unproved oil and gas properties		165,000			165,000
Construction in process and equipment deposits		1,108,054			762,884
		15,594,782			14,525,075
Less accumulated depreciation		(9,416,227	)		(9,190,327	)
	$	6,178,555		$	5,334,748

The estimated useful lives primarily range from five to 25 years for land improvements, four to 40 years for buildings and improvements and two to 15 years for machinery and equipment. The useful life for proved oil and gas properties is based on the unit-of-production method and varies by well.

In the third quarter of 2018, due to the deteriorating natural gas pricing environment at our sales point in the Piceance Basin, Nucor determined a triggering event had occurred and performed an impairment analysis that resulted in a $110.0 million non-cash impairment charge relating to two of its three groups (“fields”) of wells. One of the main assumptions that most significantly affects the undiscounted cash flows determination is management’s estimate of future pricing of natural gas and natural gas liquids. The pricing used in the impairment assessment was developed by management based on projected natural gas market supply and demand dynamics, in conjunction with a review of projections by market analysts. Management also makes key estimates on the expected reserve levels and on the expected lease operating costs. The impairment assessment was performed on each of Nucor’s three fields of wells, with each field defined by common geographic location.

The natural gas pricing environment continued to decline in 2019 and the resulting decrease in performance of the natural gas well assets reached such a point in the fourth quarter of 2019 that Nucor determined a triggering event had occurred. Nucor performed an impairment analysis on all three fields. The field of wells that was not impaired as a result of the 2018 analysis did not pass the 2019 undiscounted cash flow impairment analysis. An after-tax discounted cash flow analysis was performed for this field to determine the amount of impairment, which was $35.0 million. An increase in the estimated lease operating cost projections was the primary factor in causing this field of wells to be impaired. The non-cash impairment charges are included in the raw materials segment and in impairment of assets in the consolidated statements of earnings for the years ended December 31, 2019 and 2018, respectively. The post-impairment carrying value of this field was $12.3 million at December 31, 2019 ($51.8 million at December 31, 2018). The two previously impaired fields had a combined carrying value of $66.6 million at December 31, 2019 ($71.0 million at December 31, 2018). Changes in the natural gas industry or a prolonged low-price environment beyond what had already been assumed in the assessments could cause management to revise the natural gas and natural gas liquids price assumptions, the estimated reserves or the estimated lease operating costs. Unfavorable revisions to these assumptions or estimates could possibly result in further impairment of some or all of the fields of proved well assets.

In the steel mills segment, Nucor recorded a non-cash impairment charge of $20.0 million related to certain property, plant and equipment at our plate mill in Texas. This charge is included in impairment of assets in the consolidated statement of earnings for the year ended December 31, 2019.