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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, 2013 | Dec. 31, 2012
--- | --- | ---
Fair Value Measurement [Line Items] |  |
Assets | $ 3,126,676 | $ 3,180,095
Liabilities |  | 278
Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 543,487 | 738,408
Liabilities |  | 175
Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,534,721 | 2,387,580
Liabilities |  | 103
Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 48,468 | 54,107
Liabilities |  | 0
Short-term bonds [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 62,484 | 24,530
Short-term bonds [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 39,996 | 0
Short-term bonds [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 22,488 | 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 | 188,965 | 270,123
Money market instruments [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 188,965 | 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
Equity contracts [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities |  | 175
Equity contracts [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities |  | 175
Equity contracts [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities |  | 0
Equity contracts [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities |  | 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
Total Return Swap [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 346 |
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 | 346 |
Total Return Swap [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 |
Fixed maturity securities [Member] | U.S. government bonds and agencies [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 16,128 | 14,204
Fixed maturity securities [Member] | U.S. government bonds and agencies [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 16,128 | 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 | 2,149,615 | 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 | 2,149,615 | 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 | 36,096 | 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 | 36,096 | 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 | 300,331 | 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 | 300,331 | 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 | 36,344 | 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 | 36,344 | 42,801
Equity securities [Member] | Public utilities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 107,102 | 85,106
Equity securities [Member] | Public utilities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 107,102 | 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 | 9,963 | 22,166
Equity securities [Member] | Banks, trusts and insurance companies [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 9,963 | 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 | 181,333 | 346,809
Equity securities [Member] | Energy and other [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 181,333 | 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 | 25,845 | 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 | 25,845 | 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 Private Credit Fund [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 12,124 | 11,306
Equity securities [Member] | Partnership Interest In Private Credit Fund [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Partnership Interest In Private Credit Fund [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity securities [Member] | Partnership Interest In Private Credit Fund [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | $ 12,124 | $ 11,306

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
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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