Property, Plant and Equipment

12 Months Ended

Dec. 31, 2018

Property, Plant and Equipment [Abstract]

Property, Plant and Equipment

Property, Plant and Equipment:

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

	Useful Lives (Years)	December 31,
		2018		2017
Land	—	$	123,518	$	118,428
Land improvements	10 – 30	63,349		63,328
Buildings and improvements	10 – 45	251,980		245,482
Machinery and equipment(a)	2 – 45	2,780,478		2,627,667
Long-term mineral rights and production equipment costs	7 – 60	696,033		675,832
Construction in progress	—	883,705		393,598
Total(b)		$	4,799,063	$	4,124,335

(a) Consists primarily of (1) short-lived production equipment components, office and building equipment and other equipment with estimated lives ranging 2 – 7 years, (2) production process equipment (intermediate components) with estimated lives ranging 8 – 19 years, (3) production process equipment (major unit components) with estimated lives ranging 20 – 29 years, and (4) production process equipment (infrastructure and other) with estimated lives ranging 30 – 45 years.

(b) As of December 31, 2017, $215.9 million of Property, plant and equipment, at cost, was classified as Assets held for sale in the consolidated balance sheets. See Note 3, “Divestitures,” for additional information.

The cost of property, plant and equipment is depreciated generally by the straight-line method. Depletion of long-term mineral rights is based on the units-of-production method. Depreciation expense, including depletion, amounted to $170.0 million, $169.5 million and $178.8 million during the years ended December 31, 2018, 2017 and 2016, respectively. Depreciation expense related to discontinued operations was $8.9 million during the year ended December 31, 2016. Interest capitalized on significant capital projects in 2018, 2017 and 2016 was $19.3 million, $7.4 million and $6.8 million, respectively.