(Loss) Earnings per Share (Details) (USD $) In Thousands, except 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
Retained Earnings Adjustments [Line Items]
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)
Dividends on Convertible Preferred Stock																	(11,385)	(9,962)	0
(Loss) income attributable to common stockholders																	(492,554)	107,564	(50,774)
Weighted average common shares outstanding—basic																	107,064,892	83,454,221	62,332,054
Basic (loss) earnings 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)
Effect of dilutive 3.25% Convertible Notes																	0	413	0
(Loss) income attributable to common stockholders adjusted for effect of dilution																	$ (492,554)	$ 107,977	$ (50,774)
Weighted average common shares outstanding—diluted																	107,064,892	85,220,017	62,332,054
Diluted (loss) earnings 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)
Convertible Debt | 3.25% Convertible Notes
Retained Earnings Adjustments [Line Items]
Interest rate	3.25%																3.25%
Convertible Debt | 6.00% Convertible Notes
Retained Earnings Adjustments [Line Items]
Interest rate	6.00%																6.00%

[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.