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Condensed Financial Information of Registrant Condensed Financial Information of Registrant (Narratives) (Details) (USD $)	0 Months Ended	12 Months Ended
	Jul. 02, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 03, 2014
Condensed Financial Statements, Captions [Line Items]
Proceeds from Dividends Received		$ 225,000,000us-gaap_ProceedsFromDividendsReceived	$ 120,000,000us-gaap_ProceedsFromDividendsReceived	$ 145,000,000us-gaap_ProceedsFromDividendsReceived
Capital contributions to subsidiaries		125,000mcy_CapitalContributionsToSubsidiaries	125,000mcy_CapitalContributionsToSubsidiaries	125,000mcy_CapitalContributionsToSubsidiaries
Credit facility amount		150,000,000us-gaap_LineOfCreditFacilityAmountOutstanding
Total borrowing guaranteed		20,000,000us-gaap_SecuredDebt	20,000,000us-gaap_SecuredDebt
Loan maximum borrowing capacity	200,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Revolving credit facility term	5 years
Mcc [Member]
Condensed Financial Statements, Captions [Line Items]
Credit facility amount		120,000,000us-gaap_LineOfCreditFacilityAmountOutstanding / dei_LegalEntityAxis = mcy_MccMember
Bank loan		20,000,000us-gaap_LoansPayableToBank / dei_LegalEntityAxis = mcy_MccMember
Municipal bonds, fair value		181,000,000us-gaap_MunicipalDebtSecuritiesAtCarryingValue / dei_LegalEntityAxis = mcy_MccMember
Total borrowing guaranteed		140,000,000us-gaap_SecuredDebt / dei_LegalEntityAxis = mcy_MccMember
Revolving Credit Facility [Member]
Condensed Financial Statements, Captions [Line Items]
Credit facility amount		150,000,000us-gaap_LineOfCreditFacilityAmountOutstanding / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember
Loan maximum borrowing capacity	$ 200,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember				$ 250,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_CreditFacilityAxis = us-gaap_RevolvingCreditFacilityMember
Revolving credit facility term	5 years

X

- Definition

Capital contributions to subsidiaries.

+ References

No definition available.

+ Details

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- Details

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Balance Type:	na
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X

- Definition

Amount borrowed under the credit facility as of the balance sheet date.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

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

X

- Definition

Period remaining on line of credit facility before it terminates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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Balance Type:	na
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X

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of loans from a bank with maturities initially due after one year or beyond the normal operating cycle if longer.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16(a))

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

-Article 5

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	instant

X

- Definition

Bonds or similar securities issued by state, city, or local governments or the agencies operated by state, city, or local governments which are short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Cash Equivalents

-URI http://asc.fasb.org/extlink&oid=6507016

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
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X

- Definition

Dividends received on equity and other investments during the current period.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 25

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3536-108585

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- Definition

Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878



Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9



Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 20, 22

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-Publisher FASB

-Name Accounting Standards Codification

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-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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