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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 | Mar. 31, 2014 | Dec. 31, 2013
--- | --- | ---
Fair Value Measurement [Line Items] |  |
Assets | $ 3,318,203 | $ 3,159,962
Liabilities | 426 | 140
Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 644,226 | 558,750
Liabilities | 426 | 140
Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 2,661,251 | 2,582,712
Liabilities | 0 | 0
Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 12,726 | 16,850
Liabilities | 0 | 0
Short-term bonds [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 86,866 | 52,888
Short-term bonds [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 69,986 | 39,998
Short-term bonds [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 16,880 | 12,890
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 | 216,694 | 262,888
Money market instruments [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 216,694 | 262,888
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 | 1,499 | 1,650
Total Return Swap [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Total Return Swap [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 1,499 | 1,650
Total Return Swap [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 0 | 0
Equity contracts [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 426 | 140
Equity contracts [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Liabilities | 426 | 140
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
Fixed maturity securities [Member] | U.S. government bonds and agencies [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 16,132 | 16,096
Fixed maturity securities [Member] | U.S. government bonds and agencies [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 16,132 | 16,096
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 | 2,292,975 | 2,235,323
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 | 2,292,975 | 2,235,323
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 | 39,649 | 40,247
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 | 39,649 | 40,247
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 | 284,476 | 264,685
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 | 284,476 | 264,685
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
Fixed maturity securities [Member] | Collateralized Debt Obligations [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets |  | 0
Fixed maturity securities [Member] | Collateralized Debt Obligations [Member] | Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets |  | 0
Fixed maturity securities [Member] | Collateralized Debt Obligations [Member] | Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets |  | 4,302
Equity securities [Member] | Public utilities [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 88,471 | 85,287
Equity securities [Member] | Public utilities [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 88,471 | 85,287
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 | 7,412 | 2,927
Equity securities [Member] | Banks, trusts and insurance companies [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 7,412 | 2,927
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 | 245,531 | 151,554
Equity securities [Member] | Energy and other [Member] | Level 1 [Member] |  |
Fair Value Measurement [Line Items] |  |
Assets | 245,531 | 151,554
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 | 25,772 | 29,567
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 | 25,772 | 29,567
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 | 12,726 | 12,548
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 | $ 12,726 | $ 12,548

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