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Fair Value Measurement (Summary Of Changes In Fair Value Of Level 3 Financial Assets And Financial Liabilities Held At Fair Value) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ 4,302us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember	$ 42,801us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember
Realized gains included in earnings	(755)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember	740us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember
Purchase	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember
Sales	(3,547)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember	(30,975)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember
Settlements	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember	(8,264)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember
Ending Balance	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember	4,302us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember
The amount of total gains (losses) for period included in earnings attributable to assets still held at December 31	0mcy_AmountOfTotalLossesGainsForPeriodIncludedInEarningsAttributableToAssetsStillHeld / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember	321mcy_AmountOfTotalLossesGainsForPeriodIncludedInEarningsAttributableToAssetsStillHeld / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = us-gaap_CollateralizedDebtObligationsMember
Partnership Interest in a Private Credit Fund [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	12,548us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember	11,306us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember
Realized gains included in earnings	(829)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember	1,242us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember
Purchase	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember
Sales	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember
Settlements	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember	0us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember
Ending Balance	11,719us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember	12,548us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember
The amount of total gains (losses) for period included in earnings attributable to assets still held at December 31	$ (829)mcy_AmountOfTotalLossesGainsForPeriodIncludedInEarningsAttributableToAssetsStillHeld / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember	$ 1,242mcy_AmountOfTotalLossesGainsForPeriodIncludedInEarningsAttributableToAssetsStillHeld / us-gaap_EligibleItemOrGroupForFairValueOptionAxis = mcy_PartnershipInterestInPrivateCreditFundMember

X

- Definition

Amount of total losses gains for period included in earnings attributable to assets still held

+ References

No definition available.

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- Definition

Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)

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- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

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X

- Definition

Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

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- Definition

Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

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- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258

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