CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
CONDENSED CONSOLIDATED BALANCE SHEETS
Mineral interests, accumulated depletion (in dollars)	$ 108.8	$ 84.2
Property, plant and equipment, accumulated depreciation (in dollars)	$ 662.0	$ 614.6
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	62,481,923	62,444,905
Preferred stock, par value per share (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, Authorized shares	20,000,000	20,000,000
Preferred stock, Issued shares	0	0