Property, Plant and Equipment, at Cost (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Property, Plant and Equipment [Line Items]
Land	56,249		$ 63,153
Land improvements	49,099		52,452
Buildings and improvements	214,364		235,929
Machinery and equipment	1,443,154	[1]	1,731,247	[1]
Machinery and equipment (major plant components)	663,297	[2]	688,284	[2]
Long-term mineral rights and production equipment costs	85,888		85,514
Long-term mineral rights and production equipment costs	108,619		115,505
Total	2,620,670		$ 2,972,084
Land improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	5 years
Land improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	30 years
Buildings and improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	10 years
Buildings and improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	45 years
Machinery and equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	2 years
Machinery and equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	19 years
Machinery and equipment (major plant components) | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	20 years
Machinery and equipment (major plant components) | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	45 years
Long-term mineral rights and production equipment costs | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	7 years
Long-term mineral rights and production equipment costs | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, useful life	60 years

[1] Consists primarily of (1) short-lived production equipment components, office and building equipment and other equipment with estimated lives ranging 2 – 7 years, and (2) production process equipment (intermediate components) with estimated lives ranging 8 – 19 years.

[2] Consists primarily of (1) production process equipment (major unit components) with estimated lives ranging 20 – 29 years, and (2) production process equipment (infrastructure and other) with estimated lives ranging 30 – 45 years.