Unaudited Supplementary Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 134,259		$ 205,604		$ 104,577		$ 84,470		$ 132,905		$ 138,050		$ 99,615		$ 26,261		$ 528,910	$ 396,831	$ 35,157
Gross profit	(20,473)		10,864		(4,073)		31,027		70,272		82,392		56,692		9,584		17,345	218,941	666
(Loss) income before income taxes and equity earnings	(382,779)		(43,745)		(93,970)		(5,434)		42,607		64,157		42,143		(1,999)		(525,928)	146,910	(50,774)
Net (loss) income	(389,490)		(15,451)		(66,924)		(3,478)		26,674		45,101		48,755		(2,198)		(475,343)	118,334	(50,774)
Net (loss) income attributable to Molycorp stockholders	$ (391,196)		$ (18,891)		$ (67,604)		$ (3,478)		$ 26,579		$ 45,356		$ 47,787		$ (2,198)		$ (481,169)	$ 117,526	$ (50,774)
(Loss) income per share of common stock:
Basic (in dollars per share)	$ (3.16)	[1]	$ (0.19)	[1]	$ (0.71)	[1]	$ (0.07)	[1]	$ 0.27	[1]	$ 0.51	[1]	$ 0.54	[1]	$ (0.04)	[1]	$ (4.60)	$ 1.29	$ (0.81)
Diluted (in dollars per share)	$ (3.16)	[1]	$ (0.19)	[1]	$ (0.71)	[1]	$ (0.07)	[1]	$ 0.26	[1]	$ 0.49	[1]	$ 0.53	[1]	$ (0.04)	[1]	$ (4.60)	$ 1.27	$ (0.81)

[1] The sum of the quarterly income (loss) per share may be different than the per share amount for the year as the calculation for each quarter is based on the weighted average shares outstanding for that period.