Property, Plant, Equipment and Development (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Property, plant and equipment
Property, plant and equipment, gross	$ 2,893.6	$ 2,718.5
Less: Accumulated depreciation, depletion and amortization	(208.0)	(179.6)
Property, plant and equipment, net	2,685.6	2,538.9
Mining properties and mineral reserves
Property, plant and equipment
Property, plant and equipment, gross	976.4	978.0
Mining and milling equipment and facilities
Property, plant and equipment
Property, plant and equipment, gross	481.4	467.5
Processing facilities
Property, plant and equipment
Property, plant and equipment, gross	166.0	165.8
Construction-in-progress
Property, plant and equipment
Property, plant and equipment, gross	1,251.1	1,089.0
Construction-in-progress | Endako Mine Joint Ventrue
Property, plant and equipment
Valuation allowances and reserves, balance	69.0	55.4
Construction-in-progress | Mt. Milligan
Property, plant and equipment
Property, plant and equipment, gross	1,239.7	1,079.8
Other
Property, plant and equipment
Property, plant and equipment, gross	$ 18.7	$ 18.2
Endako Mine
Property, plant and equipment
Percentage of ownership interest in project (as a percent)		75.00%