Property, Plant and Equipment

12 Months Ended

Dec. 31, 2017

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, 2017 and 2016 (in thousands):

	Useful Lives (Years)	December 31,
		2017		2016
Land	—	$	118,428	$	120,842
Land improvements	10 – 30	63,328		59,387
Buildings and improvements	10 – 45	245,482		256,603
Machinery and equipment(a)	2 – 45	2,627,667		2,501,481
Long-term mineral rights and production equipment costs	7 – 60	675,832		654,006
Construction in progress	—	393,598		318,203
Total(b)		$	4,124,335	$	3,910,522

(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 amounted to $169.5 million, $178.8 million and $162.2 million during the years ended December 31, 2017, 2016 and 2015, respectively. Depreciation expense related to discontinued operations was $8.9 million and $18.5 million during the years ended December 31, 2016 and 2015, respectively. Interest capitalized on significant capital projects in 2017, 2016 and 2015 was $7.4 million, $6.8 million and $11.2 million, respectively.