Earnings (Loss) per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				7 Months Ended	12 Months Ended			46 Months Ended
	Mar. 31, 2012		Mar. 31, 2011		Dec. 31, 2008	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Mar. 31, 2012
Earnings (Loss) per Share
Net (loss) income attributable to Molycorp stockholders	$ (3,478)		$ (2,198)		$ (14,074)	$ 118,334	$ (50,774)	$ (28,587)	$ 21,421
Cumulative undeclared and paid dividends on Convertible Preferred Stock	(2,846)		(1,423)						(12,808)
(Loss) income attributable to common stockholders	$ (6,324)		$ (3,621)						$ 8,613
Weighted average common shares outstanding - basic	87,006,460	[1]	82,253,700	[1]					60,086,657	[1]
Basic earnings (loss) income per share	$ (0.07)		$ (0.04)						$ 0.14
Weighted average common shares outstanding - diluted	87,006,460	[1]	82,253,700	[1]					60,087,803	[1]
Diluted (loss) income per share	$ (0.07)		$ (0.04)						$ 0.14

[1] Weighted average shares outstanding include the retroactive treatment of exchange ratios for conversion of Class A common stock and Class B common stock to common stock in conjunction with the initial public offering.