Goodwill and Other Intangible Assets Narrative (Details) (USD $)	3 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Goodwill [Line Items]
Goodwill, impairment loss from initial purchase price allocation	$ 258,300,000
Impairment of goodwill	289,900,000	289,894,000	0	0
Impairment of finite-lived intangible assets		(6,000,000)
Amortization expense		28,579,000	236,000	100,000
Magnetic Materials and Alloys
Goodwill [Line Items]
Impairment of goodwill		112,694,000
Molycorp Canada
Goodwill [Line Items]
Impairment of goodwill		287,900,000
Intangibles acquired in Molycorp Canada acquisition	482,200,000	482,200,000
Goodwill, additional impairment loss from final purchase price allocation	31,600,000
Molycorp Canada | Magnetic Materials and Alloys
Goodwill [Line Items]
Impairment of finite-lived intangible assets	6,000,000
Molycorp Metals and Alloys (MMA) | Magnetic Materials and Alloys
Goodwill [Line Items]
Impairment of goodwill	2,000,000
Trade names
Goodwill [Line Items]
Additions		$ 14,800,000