CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
CONDENSED CONSOLIDATED BALANCE SHEETS
Mineral interests, accumulated depletion (in dollars)	$ 199.4	$ 179.6
Property, plant and equipment, accumulated depreciation (in dollars)	$ 845.2	$ 796.7
Common stock, par value per share (in dollars per share)	$ 0.01	$ 0.01
Common stock, Authorized shares	200,000,000	200,000,000
Common stock, Issued shares	62,563,004	62,521,300