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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 | Sep. 30, 2012 | Dec. 31, 2011
--- | --- | ---
Fair Value Measurement [Line Items] |  |
Assets | $ 3,160,613 | $ 3,062,421
Liabilities | 431 | 1,325
Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 649,127 | 600,692
Liabilities | 171 | 655
Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,419,554 | 2,404,218
Liabilities | 260 | 670
Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 91,932 | 57,511
Liabilities | 0 | 0
Short-term bonds [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 32,695 | 9,011
Short-term bonds [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Short-term bonds [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 32,695 | 9,011
Short-term bonds [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Money market instruments [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 188,387 | 227,433
Money market instruments [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 188,387 | 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] |  |
Assets | 1 |
Liabilities | 171 | 655
Equity contracts [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 1 |
Liabilities | 171 | 655
Equity contracts [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 |
Liabilities | 0 | 0
Equity contracts [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 |
Liabilities | 0 | 0
Interest rate swap agreements [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 260 | 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 | 260 | 670
Interest rate swap agreements [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 0 | 0
Fixed maturity securities [Member] | U.S. government bonds and agencies [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 12,153 | 14,298
Fixed maturity securities [Member] | U.S. government bonds and agencies [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 12,153 | 14,298
Fixed maturity securities [Member] | U.S. government bonds and agencies [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Fixed maturity securities [Member] | U.S. government bonds and agencies [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 | 2,205,136 | 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 | 2,205,136 | 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 | 32,418 | 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 | 32,418 | 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 | 137,653 | 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 | 137,653 | 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 | 81,357 | 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 | 81,357 | 47,503
Equity securities [Member] | Public utilities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 83,187 | 26,342
Equity securities [Member] | Public utilities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 83,187 | 26,342
Equity securities [Member] | Public utilities [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Public utilities [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Banks, trusts and insurance companies [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 18,546 | 16,027
Equity securities [Member] | Banks, trusts and insurance companies [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 18,546 | 16,027
Equity securities [Member] | Banks, trusts and insurance companies [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Banks, trusts and insurance companies [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 | 346,853 | 316,592
Equity securities [Member] | Energy and other [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 346,853 | 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 | 11,652 | 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 | 11,652 | 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 | 10,575 | 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 | $ 10,575 | $ 10,008

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- 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