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General - Allowance for Credit Losses on Reinsurance Recoverables (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward] |  |
Beginning balance | $ 0 | $ 0
Provision during the period for expected credit losses | (11) | 0
Write-off amounts during the period | 0 | 0
Recoveries during the period of amounts previously written off | 0 | 0
Ending balance | 148 | 0
Cumulative Effect, Period Of Adoption, Adjustment [Member] |  |
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward] |  |
Beginning balance | 159 | 0
Cumulative Effect, Period Of Adoption, Adjusted Balance [Member] |  |
Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward] |  |
Beginning balance | $ 159 | $ 0

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- Definition Reinsurance Recoverable, Allowance for Credit Loss, Recovery + References No definition available. + Details
Name: | mcy_ReinsuranceRecoverableAllowanceforCreditLossRecovery
Namespace Prefix: | mcy_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward] + References No definition available. + Details
Name: | mcy_ReinsuranceRecoverableAllowanceforCreditLossRollForward
Namespace Prefix: | mcy_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Reinsurance Recoverable, Credit Loss Expense (Reversal) + References No definition available. + Details
Name: | mcy_ReinsuranceRecoverableCreditLossExpenseReversal
Namespace Prefix: | mcy_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of the write-down or write-off of a receivable from a reinsurer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14326-108349 + Details
Name: | us-gaap_ReinsuranceLossOnUncollectibleAccountsInPeriodAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Amount of valuation allowance for reinsurance recoverables. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References No definition available. + Details
Name: | us-gaap_ReinsuranceRecoverablesAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | mcy_CumulativeEffectPeriodOfAdoptionAxis=mcy_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | mcy_CumulativeEffectPeriodOfAdoptionAxis=mcy_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |