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Investments (Estimated Fair Value Of Investments) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Investments [Abstract]
Due in one year or less	$ 30,756
Due after one year through five years	370,609
Due after five years through ten years	556,519
Due after ten years	1,450,334
Mortgage-backed securities	37,371
Total	$ 2,445,589
Percentage of fixed maturities	3.70%

X

- Definition

Mortgage backed securities trading securities fair value disclosure.

+ References

No definition available.

+ Details

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Namespace Prefix:	mcy_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Percentage of fixed maturities rated below investment grade and non-rated.

+ References

No definition available.

+ Details

Name:	mcy_PercentageOfFixedMaturitySecurities
Namespace Prefix:	mcy_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Trading securities debt maturities after five through ten years fair value.

+ References

No definition available.

+ Details

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Namespace Prefix:	mcy_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Trading securities debt maturities after one through five years fair value.

+ References

No definition available.

+ Details

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Namespace Prefix:	mcy_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Trading securities debt maturities after ten years fair value.

+ References

No definition available.

+ Details

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Namespace Prefix:	mcy_
Data Type:	xbrli:monetaryItemType
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Period Type:	instant

X

- Definition

Trading securities debt maturities within one year fair value.

+ References

No definition available.

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Namespace Prefix:	mcy_
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- Details

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Balance Type:	na
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X

- Definition

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents investment securities as of the balance sheet date which may include marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594



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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558



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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Trading Securities

-URI http://asc.fasb.org/extlink&oid=6526789



Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.



Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Article 7

-Section 03

-Paragraph 1

-Subparagraph a, f, g



Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6956420&loc=d3e24584-111560

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