Acquisitions (Details) (USD $)	3 Months Ended		7 Months Ended	12 Months Ended			46 Months Ended	1 Months Ended	3 Months Ended			1 Months Ended			3 Months Ended			1 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2008	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Mar. 31, 2012	Oct. 31, 2011 Molycorp Silmet	Mar. 31, 2012 Molycorp Silmet metrictonne	Oct. 24, 2011 Molycorp Silmet	Apr. 01, 2011 Molycorp Silmet	Apr. 30, 2011 Molycorp Silmet AS Silmet Grupp D	Apr. 01, 2011 Molycorp Silmet AS Silmet Grupp	Apr. 01, 2011 Molycorp Silmet Treibacher Industrie AG	Mar. 31, 2012 Molycorp Silmet Customer relationships Y	Mar. 31, 2011 MMA	Apr. 15, 2011 MMA	Apr. 30, 2011 MMA Santoku Y
Acquisitions
Interest acquired (as a percent)										9.977%	90.023%		80.00%	10.023%
Purchase consideration:
Cash consideration										$ 10,000,000	$ 9,021,000			$ 9,000,000			$ 17,500,000
Fair value of common stock											72,653,000
Total purchase consideration											81,674,000						17,500,000
The fair values of the assets and liabilities acquired:
Cash											105,000						6,395,000
Accounts receivable and other current assets											8,626,000						5,474,000
Inventory											37,404,000						11,327,000
Property and equipment, net											63,393,000						4,512,000
Intangible assets subject to amortization											2,669,000
Goodwill											1,455,000						1,977,000
Liabilities											(19,974,000)						(8,989,000)
Deferred tax liabilities																	(3,196,000)
Long-term debt											(3,184,000)
Noncontrolling interest											(8,820,000)
Total purchase consideration											81,674,000						17,500,000
Fair value of trade receivable acquired											5,000,000						4,900,000
Weighted average useful life (in years)															15
Actuals
Revenue	84,470,000
Net (loss) income	(3,478,000)	(2,198,000)	(14,074,000)	118,334,000	(50,774,000)	(28,587,000)	21,421,000
Basic earnings (loss) per share (in dollars per share)	$ (0.07)	$ (0.04)					$ 0.14
Unaudited proforma
Revenue		59,734,000
Net Income (loss)		4,916,000
Net Income (loss) Attributable To Molycorp		4,623,000
EPS Basic (in dollars per share)		$ 0.04
Other actual and pro forma information
Purchase price variance capitalized																1,100,000
Other disclosures
Common stock issued to acquire the entity (in shares)												1,593,419
Contractual value of shares issued to acquire the entity												80,000,000
Number of consecutive trading days considered for calculation of closing price												20
Discount percentage deducted from common stock price												23.00%
Increase in rare earth production capacity (in mt)									3,000
Adjustment to Additional Paid-In Capital								$ 400,000
Period of marketing and distribution agreement (in years)																		5
Period of rare earth products purchase and supply agreement (in years)																		2