Debt and Capital Lease Obligations (Details) (USD $)	12 Months Ended			7 Months Ended	12 Months Ended	7 Months Ended		12 Months Ended					0 Months Ended		12 Months Ended					0 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Common Stock	Dec. 31, 2012 Common Stock	Dec. 31, 2012 Convertible Subordinated Debt 5.00% Debentures, net of discount	Jun. 12, 2012 Convertible Subordinated Debt 5.00% Debentures, net of discount	Dec. 31, 2012 Notes Payable to Banks Bank loans 2.69% - 3.88% due February 2012 - September 2017	Dec. 31, 2011 Notes Payable to Banks Bank loans 2.69% - 3.88% due February 2012 - September 2017	Dec. 31, 2012 Notes Payable to Banks bank loans with a weighted avergare rate of 3.57% due January 2013 - Septmber 2017	Dec. 31, 2012 Notes Payable to Banks Molycorp Canada	Dec. 31, 2012 Notes Payable to Banks Molycorp Canada and Molycorp Silmet bank loans with a weighted avergare rate of 3.57% due January 2013 - Septmber 2017	May 25, 2012 Secured Debt 10% Senior Securied Notes, net of discount, due June 2020	Dec. 31, 2012 Secured Debt 10% Senior Securied Notes, net of discount, due June 2020	Dec. 31, 2012 Secured Debt Before June 1, 2016	Dec. 31, 2012 Secured Debt After June 1, 2016	Dec. 31, 2012 Secured Debt Before June 1, 2015	Dec. 31, 2012 Convertible Debt 6.00% Convertible Notes	Aug. 22, 2012 Convertible Debt 6.00% Convertible Notes	Jun. 15, 2011 Convertible Debt 3.25% Convertible Notes	Dec. 31, 2012 Convertible Debt 3.25% Convertible Notes	Dec. 31, 2011 Convertible Debt 3.25% Convertible Notes
Debt Instrument [Line Items]
Interest rate						5.00%	5.00%			3.57%				10.00%				6.00%			3.25%
Debt Instrument, Interest Rate, Effective Percentage																			4.60%	3.95%
Convertible carrying amount of liability component																		$ 332,000,000			$ 198,700,000	$ 190,900,000
Amortization of debt discount	2,544,000	674,000	0															5,300,000			7,800,000	3,700,000
Debt instrument, minimum interest rate								2.69%
Debt instrument, maximum interest rate								3.88%
Convertible debt instrument, conversion price							$ 13.80											$ 12		$ 71.40
debt instrument, convertible, conversion face amount						9,400,000
Repurchased face amount						217,900,000
Repurchase amount						8,000,000
Stock issued during the period, shares				13,800,000	99,723
Percentage of last reported sale price of Molycorp common stock over conversion price on or after September 1, 2015 giving right to the Notes redemption																		130.00%
Trading days during any 30 consecutive trading day period ending on, and including, the trading day immediately preceding the date on which Molycorp provides notice of redemption																		20 days
Consecutive trading day period ending on, and including, the trading day immediately preceding the date on which Molycorp provides notice of redemption																		30 days
redemption price of principal amount																		100.00%
Convertible debt instrument, carrying amount of equity component																		68,700,000				36,200,000
Convertible interest expense						2,300,000												14,200,000			15,300,000	7,500,000
Number of borrowed shares				13,800,000	13,800,000
Debt instrument, redeemable debt, redemption percentage of principal amount															100.00%	105.00%
Debt instrument, redeemable debt, redeemable principal from proceeds from equity																	35.00%
Debt instrument, redeemable debt, redeemable percentage of principal from proceed from equity																	110.00%
Fair market value of assets excluded from security interest, maximum														15,000,000
Cash collateral excluded from security interest, maximum														105.00%
Voting stock of foreign subsidiaries excluded from security interest, minimum														65.00%
Debt instrument, face amount							230,000,000						650,000,000					54,000,000	360,000,000	230,000,000
Proceeds from issuance of long-term debt													635,400,000					395,700,000		223,100,000
6.00% Convertible Noted purchased by certain directors, officers and other related parties of Molycorp																		6,400,000
Convertible conversion ratio																		83.333		14.0056
Principal amount convertible into common stock																		1,000
Current portion of debt outstanding	39,252,000	1,516,000				0			1,516,000			39,252,000		0				0			0	0
Weighted average interest rate											3.68%
Noncurrent portion of debt outstanding	1,173,669,000	196,545,000				2,774,000			5,668,000			4,118,000		636,111,000				331,977,000			198,689,000	190,877,000
Long-term debt, gross						2,800,000
Short-term debt											37,900,000
Long-term purchase commitment, monthly purchase amount	400,000
Capital Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
Total capital leases payments due	48,300,000
Due in 2013	5,100,000
Due in 2014	5,100,000
Due in 2015	5,100,000
Due in 2016	5,100,000
Due in 2017	5,100,000
Due thereafter	22,900,000
Executory costs	1,900,000
Imputed interest	30,900,000
Unused portion of line of credit	50,500,000
Capital lease obligations, current	352,000
Capital lease obligations, Noncurrent	15,163,000
Total debt and Capital lease obligations, Current	39,604,000	1,516,000
Total debt and Capital lease obligations, Non-Current	1,188,832,000	196,545,000
Long-term Purchase Commitment, Period (in years)	10 years
Long-term Debt, by Maturity [Abstract]
2013	39,300,000
2014	1,500,000
2015	1,500,000
2016	230,800,000
2017	417,200,000
Thereafter	$ 650,000,000