Commitments and Contingencies Purchase Commitments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012
Commitments and Contingencies Disclosure [Abstract]
Purchase obligations and other commitment, Total	$ 293,383
Purchase obligations and other commitment, Less Than 1 Year	265,727
Purchase obligations and other commitment, 1 - 3 Years	14,017
Purchase obligations and other commitment, 4 - 5 Years	7,131
Purchase obligation and other comitment, More Than Five Years	$ 6,508

Unrecorded Unconditional Purchase Obligation Due in Years Four and Five

No definition available.

Name:	mcp_UnrecordedUnconditionalPurchaseObligationDueInYearsFourAndFive
Namespace Prefix:	mcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Unrecorded Unconditional Purchase Obligation Due in Years One to Three

+ References

No definition available.

+ Details

Name:	mcp_UnrecordedUnconditionalPurchaseObligationDueInYearsOneToThree
Namespace Prefix:	mcp_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_CommitmentsAndContingenciesDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the next fiscal year following the latest fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 440

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6394976&loc=d3e25336-109308

+ Details

Name:	us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of the unrecorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing after the fifth fiscal year following the latest fiscal year.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant