Segment Information - Schedule of Revenues by Segment (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended		12 Months Ended
	Dec. 31, 2010	Jun. 30, 2010	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Internally allocated interest costs, pre tax rate	6.50%	7.15%	6.00%	6.25%
Capital structure assumption, debt portion			50
Capital structure assumption, equity portion			50
Mineral rights	$ 15.8		$ 13.4	$ 15.0