Property, Plant and Equipment (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Cost	$ 21,150,193	$ 17,336,643
Property, plant and equipment, Net	13,011,606	10,475,149
Mineral Rights
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Cost	1,051,153	1,078,770
Property, plant and equipment, Net	556,399	619,950
Administrative assets and support equipment
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Net	$ 26,611	$ 21,154