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Statutory Balances and Accounting Practices (Schedule Of Statutory Net Income And Capital And Surplus) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Statutory Accounting Practices [Line Items]      
Statutory net income $ 155,654us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount [1] $ 235,251us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount [1] $ 63,365us-gaap_StatutoryAccountingPracticesStatutoryNetIncomeAmount [1]
Statutory capital and surplus $ 1,438,281us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance $ 1,528,682us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance $ 1,440,973us-gaap_StatutoryAccountingPracticesStatutoryCapitalAndSurplusBalance
RBC authorized control level 800.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets 500.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets 500.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
Minimum RBC authorized control level 200.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets 200.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets 200.00%us-gaap_TierOneRiskBasedCapitalRequiredForCapitalAdequacyToRiskWeightedAssets
CAIC [Member]      
Statutory Accounting Practices [Line Items]      
RBC authorized control level 386.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= mcy_CAICMember		 520.00%us-gaap_TierOneRiskBasedCapitalToRiskWeightedAssets
/ dei_LegalEntityAxis
= mcy_CAICMember		  
[1] Statutory net income excludes changes in the fair value of the investment portfolio as a result of the application of the fair value option under GAAP, and reflects the impacts of other differences from GAAP.