Summary of significant accounting policies (Details) (USD $)	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	Dec. 31, 2009
Property, Plant and Equipment [Line Items]
Allowance for doubtful accounts	$ 2,600,000								$ 0								$ 2,600,000	$ 0
Impairment of finite-lived intangible assets																	(6,000,000)
Impairment of long-lived assets to be disposed of	5,900,000																5,900,000		3,100,000
Impairment of goodwill	289,900,000																289,894,000	0	0
Valuation allowance	(20,631,000)								0								(20,631,000)	0
Investments	64,036,000								20,000,000								64,036,000	20,000,000
Deferred tax assets	0																0
Total non-current assets	2,358,857,000								616,081,000								2,358,857,000	616,081,000
Total assets	2,992,535,000								1,255,125,000								2,992,535,000	1,255,125,000
Deferred tax liabilities	160,675,000								18,899,000								160,675,000	18,899,000
Total non-current liabilities	1,381,860,000								231,272,000								1,381,860,000	231,272,000
Additional paid-in capital	1,691,429,000								838,547,000								1,691,429,000	838,547,000
Deficit	(466,091,000)								15,078,000								(466,091,000)	15,078,000
Total Molycorp stockholders' equity	1,216,046,000								845,230,000								1,216,046,000	845,230,000
Noncontrolling interests	35,212,000								0								35,212,000	0
Total stockholders’ equity	1,251,258,000								845,230,000								1,251,258,000	845,230,000	446,513,000	74,615,000
Impairment of goodwill and other long-lived assets																	301,755,000	0	3,100,000
Loss before income taxes and equity earnings	(382,779,000)		(43,745,000)		(93,970,000)		(5,434,000)		42,607,000		64,157,000		42,143,000		(1,999,000)		(525,928,000)	146,910,000	(50,774,000)
Net loss	(389,490,000)		(15,451,000)		(66,924,000)		(3,478,000)		26,674,000		45,101,000		48,755,000		(2,198,000)		(475,343,000)	118,334,000	(50,774,000)
Net loss attributable to Molycorp stockholders	(391,196,000)		(18,891,000)		(67,604,000)		(3,478,000)		26,579,000		45,356,000		47,787,000		(2,198,000)		(481,169,000)	117,526,000	(50,774,000)
Comprehensive loss																	(476,295,000)	109,853,000	(50,774,000)
Comprehensive loss attributable to Molycorp stockholders																	(470,469,000)	109,468,000	(50,774,000)
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)
As originally reported
Property, Plant and Equipment [Line Items]
Investments	65,126,000																65,126,000
Deferred tax assets	1,083,000																1,083,000
Total non-current assets	2,361,030,000																2,361,030,000
Total assets	2,994,708,000																2,994,708,000
Deferred tax liabilities	166,215,000																166,215,000
Total non-current liabilities	1,387,400,000																1,387,400,000
Additional paid-in capital	1,680,838,000																1,680,838,000
Deficit	(434,476,000)																(434,476,000)
Total Molycorp stockholders' equity	1,237,070,000																1,237,070,000
Noncontrolling interests	10,821,000																10,821,000
Total stockholders’ equity	1,247,891,000																1,247,891,000
Impairment of goodwill and other long-lived assets																	270,140,000
Loss before income taxes and equity earnings																	(494,313,000)
Net loss																	(443,728,000)
Net loss attributable to Molycorp stockholders																	(449,554,000)
Comprehensive loss																	(444,680,000)
Comprehensive loss attributable to Molycorp stockholders																	(438,854,000)
Basic (in dollars per share)																	$ (4.31)
Diluted (in dollars per share)																	$ (4.31)
Foreign Currency Gain (Loss)
Property, Plant and Equipment [Line Items]
Intercompany foreign currency translation loss																	200,000	2,700,000
Molycorp Canada
Property, Plant and Equipment [Line Items]
Impairment of goodwill																	287,900,000
Molycorp Canada | Revision
Property, Plant and Equipment [Line Items]
Investments	(1,090,000)																(1,090,000)
Deferred tax assets	(1,083,000)																(1,083,000)
Total non-current assets	(2,173,000)																(2,173,000)
Total assets	(2,173,000)																(2,173,000)
Deferred tax liabilities	(5,540,000)																(5,540,000)
Total non-current liabilities	(5,540,000)																(5,540,000)
Additional paid-in capital	10,591,000																10,591,000
Deficit	(31,615,000)																(31,615,000)
Total Molycorp stockholders' equity	(21,024,000)																(21,024,000)
Noncontrolling interests	24,391,000																24,391,000
Total stockholders’ equity	3,367,000																3,367,000
Impairment of goodwill and other long-lived assets																	31,615,000
Loss before income taxes and equity earnings																	(31,615,000)
Net loss																	(31,615,000)
Net loss attributable to Molycorp stockholders																	(31,615,000)
Comprehensive loss																	(31,615,000)
Comprehensive loss attributable to Molycorp stockholders																	(31,615,000)
Basic (in dollars per share)																	$ (0.29)
Diluted (in dollars per share)																	$ (0.29)
Magnetic Materials and Alloys
Property, Plant and Equipment [Line Items]
Impairment of goodwill																	112,694,000
Total assets	593,197,000								30,061,000								593,197,000	30,061,000
Loss before income taxes and equity earnings																	(126,981,000)	2,336,000
Magnetic Materials and Alloys | Molycorp Canada
Property, Plant and Equipment [Line Items]
Impairment of finite-lived intangible assets	6,000,000
Resources
Property, Plant and Equipment [Line Items]
Impairment of long-lived assets to be disposed of	3,800,000
Total assets	1,802,842,000								824,712,000								1,802,842,000	824,712,000
Loss before income taxes and equity earnings																	(70,469,000)	218,391,000
Rare Metals
Property, Plant and Equipment [Line Items]
Impairment of long-lived assets to be disposed of	2,000,000
Impairment of goodwill																	16,068,000
Total assets	117,961,000								71,634,000								117,961,000	71,634,000
Loss before income taxes and equity earnings																	(19,918,000)	(634,000)
Customer relationships
Property, Plant and Equipment [Line Items]
Impairment of finite-lived intangible assets																	0
Chinese Land Use Rights
Property, Plant and Equipment [Line Items]
Finite-lived intangible asset useful life																	50 years
Impairment of finite-lived intangible assets																	0
Rare earth quotas
Property, Plant and Equipment [Line Items]
Impairment of finite-lived intangible assets																	0
Patents
Property, Plant and Equipment [Line Items]
Impairment of finite-lived intangible assets																	(6,000,000)
Trade names
Property, Plant and Equipment [Line Items]
Impairment of finite-lived intangible assets																	$ 0
Minimum | Customer relationships
Property, Plant and Equipment [Line Items]
Finite-lived intangible asset useful life																	2 years
Minimum | Rare earth quotas
Property, Plant and Equipment [Line Items]
Finite-lived intangible asset useful life																	2 years
Minimum | Patents
Property, Plant and Equipment [Line Items]
Finite-lived intangible asset useful life																	2 years
Minimum | Trade names
Property, Plant and Equipment [Line Items]
Finite-lived intangible asset useful life																	2 years
Maximum | Customer relationships
Property, Plant and Equipment [Line Items]
Finite-lived intangible asset useful life																	16 years
Maximum | Rare earth quotas
Property, Plant and Equipment [Line Items]
Finite-lived intangible asset useful life																	16 years
Maximum | Patents
Property, Plant and Equipment [Line Items]
Finite-lived intangible asset useful life																	16 years
Maximum | Trade names
Property, Plant and Equipment [Line Items]
Finite-lived intangible asset useful life																	16 years
Buildings and improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful life																	4 years
Buildings and improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful life																	40 years
Plant and equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful life																	2 years
Plant and equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment useful life																	15 years

[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.