Name:	merc_ShareBasedCompensationArrangementByShareBasedPaymentAwardLiabilityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix:	merc_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

Share based compensation arrangement by share based payments award liability instruments other than options.

No definition available.

Name:	merc_ShareBasedCompensationArrangementByShareBasedPaymentsAwardLiabilityInstrumentsOtherThanOptionsAbstract
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Share-based compensation arrangement by share-based payments award,liability instruments other than options, grants in period

No definition available.

Name:	merc_ShareBasedCompensationArrangementByShareBasedPaymentsAwardLiabilityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	merc_
Data Type:	xbrli:sharesItemType
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Share based compensation arrangement by share based payments award liability instruments other than options vested in period.

No definition available.

Name:	merc_ShareBasedCompensationArrangementByShareBasedPaymentsAwardLiabilityInstrumentsOtherThanOptionsVestedInperiod
Namespace Prefix:	merc_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 718
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(ii)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480429/718-10-50-2

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Period Type:	duration

The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
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Data Type:	na
Balance Type:
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Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockUnitsRSUMember
Namespace Prefix:
Data Type:	na
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Name:	us-gaap_AwardTypeAxis=merc_EquityDeferredStockUnitsMember
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