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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 | Dec. 31, 2013 | Dec. 31, 2012
--- | --- | ---
Fair Value Measurement [Line Items] |  |
Assets | $ 3,159,962 | $ 3,180,095
Liabilities | 140 | 278
Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 558,750 | 738,408
Liabilities | 140 | 175
Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,584,362 | 2,387,580
Liabilities | 0 | 103
Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 16,850 | 54,107
Liabilities | 0 | 0
Short-term bonds [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 52,888 | 24,530
Short-term bonds [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 39,998 | 0
Short-term bonds [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 12,890 | 24,530
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 | 262,888 | 270,123
Money market instruments [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 262,888 | 270,123
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
Total Return Swap [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 1,650 |
Total Return Swap [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 |
Total Return Swap [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 1,650 |
Total Return Swap [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 |
Equity contracts [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 140 | 175
Equity contracts [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 140 | 175
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 |  | 103
Interest rate swap agreements [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities |  | 0
Interest rate swap agreements [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities |  | 103
Interest rate swap agreements [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities |  | 0
Fixed Maturity Securities [Member] | U.S. government bonds and agencies [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 16,096 | 14,204
Fixed Maturity Securities [Member] | U.S. government bonds and agencies [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 16,096 | 14,204
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 | 2,235,323 | 2,165,095
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 | 2,235,323 | 2,165,095
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 | 40,247 | 30,703
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 | 40,247 | 30,703
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 | 264,685 | 155,551
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 | 264,685 | 155,551
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 | 4,302 | 42,801
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 | 4,302 | 42,801
Equity Securities [Member] | Public utilities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 85,287 | 85,106
Equity Securities [Member] | Public utilities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 85,287 | 85,106
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 | 2,927 | 22,166
Equity Securities [Member] | Banks, trusts and insurance companies [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,927 | 22,166
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 | 151,554 | 346,809
Equity Securities [Member] | Energy and other [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 151,554 | 346,809
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 | 29,567 | 11,701
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 | 29,567 | 11,701
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 | 12,548 | 11,306
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 | $ 12,548 | $ 11,306

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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
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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