	(in thousands)
December 31,	2020			2019
Land and improvements, net	$	744,305		$	719,736
Buildings and improvements		1,505,913			1,413,690
Machinery and equipment		12,204,738			11,630,179
Proved oil and gas properties		558,231			558,123
Leasehold interest in unproved oil and gas properties		138,000			165,000
Construction in process and equipment deposits		1,603,416			1,108,054
		16,754,603			15,594,782
Less accumulated depreciation		(9,855,493	)		(9,416,227	)
	$	6,899,110		$	6,178,555

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 2019, 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 losses and impairments of assets in the consolidated statement of earnings for the year ended December 31, 2019.

In 2020, Nucor recorded non-cash impairment charges totaling $103.2 million related to certain inventory and long-lived assets, which primarily related to our Castrip sheet mill operations. Due to the advancements in the capabilities at our new cold mill and galvanizing line we have under construction at Nucor Steel Arkansas, we believe the value of the technology and process has diminished for Nucor. As such, the existing Castrip assets are not expected to be materially utilized going forward. These charges are included in losses and impairments of assets in the consolidated statement of earnings for the year ended December 31, 2020.

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 $110.0 million of non-cash impairment charges relating to two of its three groups (“fields”) of wells. In the fourth quarter of 2019, due to the deteriorating natural gas pricing environment at our sales point in the Piceance Basin as well as the decreased performance of the natural gas well assets, Nucor determined a triggering event had occurred and performed an impairment analysis on all three fields of wells. As a result of the fourth quarter of 2019 analysis, a $35.0 million non-cash impairment charge was recorded on the field of wells that was not previously impaired in the third quarter of 2018. 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 losses and impairments of assets in the consolidated statements of earnings for the years ended December 31, 2019 and 2018.

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 assessments 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 assessments were performed on each of Nucor’s three fields of wells, with each field defined by common geographic location. The combined carrying value of the three fields of wells was $71.7 million at December 31, 2020 ($78.9 million at December 31, 2019).

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 2020, regulatory authorities in Colorado adopted new rules that became effective January 2021. One of these rules increases drilling setback distances. In the fourth quarter of 2020, Nucor determined a triggering event had occurred, as we do not expect to be able to access the full extent of the resources in the ground, and performed an impairment analysis. As a result, Nucor recorded a $27.0 million non-cash impairment charge related to the write-down of our leasehold interest in unproved oil and gas properties. This charge is included in losses and impairments of assets in the consolidated statement of earnings for the year ended December 31, 2020.