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Fair Value Measurement (Schedule Of Assets And Liabilities Measured At Fair Value On A Recurring Basis Valuation Techniques) (Details) (USD $) In Thousands, unless otherwise specified | Jun. 30, 2012 | Dec. 31, 2011
--- | --- | ---
Fair Value Measurement [Line Items] |  |
Assets | $ 3,113,130 | $ 3,062,421
Liabilities | 1,042 | 1,325
Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 574,999 | 600,692
Liabilities | 643 | 655
Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,450,776 | 2,404,218
Liabilities | 399 | 670
Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 87,355 | 57,511
Liabilities | 0 | 0
Short-term Investments [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 40,792 | 9,011
Short-term Investments [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Short-term Investments [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 40,792 | 9,011
Short-term Investments [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Money market instruments [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 138,534 | 227,433
Money market instruments [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 138,534 | 227,433
Money market instruments [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Money market instruments [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity contracts [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 643 | 655
Equity contracts [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 643 | 655
Equity contracts [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 0 | 0
Equity contracts [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 0 | 0
Interest rate swap agreements [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 399 | 670
Interest rate swap agreements [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 0 | 0
Interest rate swap agreements [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 399 | 670
Interest rate swap agreements [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 0 | 0
Fixed maturity securities [Member] | US government agencies debt securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 12,553 | 14,298
Fixed maturity securities [Member] | US government agencies debt securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 12,553 | 14,298
Fixed maturity securities [Member] | US government agencies debt securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | US government agencies debt securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | Municipal securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,265,322 | 2,271,275
Fixed maturity securities [Member] | Municipal securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | Municipal securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,265,322 | 2,271,275
Fixed maturity securities [Member] | Municipal securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | Mortgage-backed securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 30,887 | 37,371
Fixed maturity securities [Member] | Mortgage-backed securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | Mortgage-backed securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 30,887 | 37,371
Fixed maturity securities [Member] | Mortgage-backed securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | Corporate securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 102,550 | 75,142
Fixed maturity securities [Member] | Corporate securities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | Corporate securities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 102,550 | 75,142
Fixed maturity securities [Member] | Corporate securities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | Collateralized debt obligations [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 76,325 | 47,503
Fixed maturity securities [Member] | Collateralized debt obligations [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | Collateralized debt obligations [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | Collateralized debt obligations [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 76,325 | 47,503
Equity securities [Member] | Public Utility, Equities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 68,991 | 26,342
Equity securities [Member] | Public Utility, Equities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 68,991 | 26,342
Equity securities [Member] | Public Utility, Equities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Public Utility, Equities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Banks, trusts and insurance, equities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 18,483 | 16,027
Equity securities [Member] | Banks, trusts and insurance, equities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 18,483 | 16,027
Equity securities [Member] | Banks, trusts and insurance, equities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Banks, trusts and insurance, equities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Energy and other [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 336,438 | 316,592
Equity securities [Member] | Energy and other [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 336,438 | 316,592
Equity securities [Member] | Energy and other [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Energy and other [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Non-redeemable preferred stock [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 11,225 | 11,419
Equity securities [Member] | Non-redeemable preferred stock [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Non-redeemable preferred stock [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 11,225 | 11,419
Equity securities [Member] | Non-redeemable preferred stock [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Partnership interest in a private credit fund [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 11,030 | 10,008
Equity securities [Member] | Partnership interest in a private credit fund [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Partnership interest in a private credit fund [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Partnership interest in a private credit fund [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | $ 11,030 | $ 10,008

X
- Definition This element represents a certain statement of financial position asset caption which represents a class of assets, or which may include an individual asset, measured at fair value on a recurring basis. + References No definition available. + Details
Name: | mcy_FairValueAssetsMeasuredOnRecurringBasis
Namespace Prefix: | mcy_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This element represents liabilities, including [financial] instruments that are classified in stockholders' equity, which are measured at fair value on a recurring basis. + References No definition available. + Details
Name: | mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
Namespace Prefix: | mcy_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Details
Name: | mcy_FairValueMeasurementLineItems
Namespace Prefix: | mcy_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration