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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, 2014
Dec. 31, 2013
Fair Value Measurement [Line Items]    
Assets $ 3,403,822mcy_FairValueAssetsMeasuredOnRecurringBasis $ 3,159,962mcy_FairValueAssetsMeasuredOnRecurringBasis
Liabilities 4,219mcy_FairValueLiabilitiesMeasuredOnRecurringBasis 140mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
Level 1 [Member]    
Fair Value Measurement [Line Items]    
Assets 742,886mcy_FairValueAssetsMeasuredOnRecurringBasis
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Liabilities 194mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Level 2 [Member]    
Fair Value Measurement [Line Items]    
Assets 2,649,217mcy_FairValueAssetsMeasuredOnRecurringBasis
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Liabilities 4,025mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Level 3 [Member]    
Fair Value Measurement [Line Items]    
Assets 11,719mcy_FairValueAssetsMeasuredOnRecurringBasis
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Liabilities 0mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Short-term bonds [Member]    
Fair Value Measurement [Line Items]    
Assets 88,361mcy_FairValueAssetsMeasuredOnRecurringBasis
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Short-term bonds [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Assets 69,999mcy_FairValueAssetsMeasuredOnRecurringBasis
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Short-term bonds [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Assets 18,362mcy_FairValueAssetsMeasuredOnRecurringBasis
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Short-term bonds [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Assets 0mcy_FairValueAssetsMeasuredOnRecurringBasis
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Money market instruments [Member]    
Fair Value Measurement [Line Items]    
Assets 284,181mcy_FairValueAssetsMeasuredOnRecurringBasis
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Money market instruments [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Assets 284,181mcy_FairValueAssetsMeasuredOnRecurringBasis
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Money market instruments [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Assets 0mcy_FairValueAssetsMeasuredOnRecurringBasis
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Money market instruments [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Assets 0mcy_FairValueAssetsMeasuredOnRecurringBasis
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Total Return Swap [Member]    
Fair Value Measurement [Line Items]    
Liabilities 4,025mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Total Return Swap [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Liabilities 0mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Total Return Swap [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Liabilities 4,025mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Fair Value Measurement [Line Items]    
Liabilities 0mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Equity contracts [Member]    
Fair Value Measurement [Line Items]    
Liabilities 194mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Equity contracts [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Liabilities 194mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Equity contracts [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Liabilities 0mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Equity contracts [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Liabilities 0mcy_FairValueLiabilitiesMeasuredOnRecurringBasis
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Interest rate swap agreements [Member]    
Fair Value Measurement [Line Items]    
Assets   262,888mcy_FairValueAssetsMeasuredOnRecurringBasis
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Interest rate swap agreements [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Assets   262,888mcy_FairValueAssetsMeasuredOnRecurringBasis
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Interest rate swap agreements [Member] | Level 2 [Member]    
Fair Value Measurement [Line Items]    
Assets   0mcy_FairValueAssetsMeasuredOnRecurringBasis
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Interest rate swap agreements [Member] | Level 3 [Member]    
Fair Value Measurement [Line Items]    
Assets   0mcy_FairValueAssetsMeasuredOnRecurringBasis
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Fixed Maturity Securities [Member] | U.S. government bonds and agencies [Member]    
Fair Value Measurement [Line Items]    
Assets 16,108mcy_FairValueAssetsMeasuredOnRecurringBasis
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Fixed Maturity Securities [Member] | U.S. government bonds and agencies [Member] | Level 1 [Member]    
Fair Value Measurement [Line Items]    
Assets 16,108mcy_FairValueAssetsMeasuredOnRecurringBasis
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Fair Value Measurement [Line Items]    
Assets 0mcy_FairValueAssetsMeasuredOnRecurringBasis
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Fair Value Measurement [Line Items]    
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Fixed Maturity Securities [Member] | Municipal securities [Member]    
Fair Value Measurement [Line Items]    
Assets 2,275,455mcy_FairValueAssetsMeasuredOnRecurringBasis
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Fair Value Measurement [Line Items]    
Assets 0mcy_FairValueAssetsMeasuredOnRecurringBasis
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Fair Value Measurement [Line Items]    
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Fair Value Measurement [Line Items]    
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Fixed Maturity Securities [Member] | Mortgage-backed securities [Member]    
Fair Value Measurement [Line Items]    
Assets 47,691mcy_FairValueAssetsMeasuredOnRecurringBasis
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Fair Value Measurement [Line Items]    
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Fair Value Measurement [Line Items]    
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Fair Value Measurement [Line Items]    
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Fixed Maturity Securities [Member] | Corporate Securities [Member]    
Fair Value Measurement [Line Items]    
Assets 256,930mcy_FairValueAssetsMeasuredOnRecurringBasis
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Fair Value Measurement [Line Items]    
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Fair Value Measurement [Line Items]    
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Fair Value Measurement [Line Items]    
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Fair Value Measurement [Line Items]    
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