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Subsequent Event (Details) - Wildfire [Member] - Subsequent Event [Member] $ in Millions | Oct. 15, 2017 USD ($) structure
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Subsequent Event [Line Items] |
Structures destroyed | structure | 100
Reinsurance recoverables | $ 10
Reinstatement premium, maximum | 19
Minimum [Member] |
Subsequent Event [Line Items] |
Pre-tax catastrophe losses | 60
Maximum [Member] |
Subsequent Event [Line Items] |
Pre-tax catastrophe losses | $ 100
Reinsurance Policy, Up To Two Hundred Million [Member] |
Subsequent Event [Line Items] |
Reinsurance retention policy, reinsured risk, percentage | 100.00%
Reinsurance retention policy, excess retention, amount reinsured | $ 190
Reinsurance retention policy, retention | $ 10
Reinsurance Policy, Above Two Hundred Million [Member] |
Subsequent Event [Line Items] |
Reinsurance retention policy, reinsured risk, percentage | 5.00%
Reinsurance retention policy, retention | $ 15
Reinsurance retention policy, lower limit | $ 200
Reinsurance retention policy, retention, percentage | 95.00%
Northern California |
Subsequent Event [Line Items] |
Structures destroyed | structure | 8,000

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- Definition Reinsurance Retention Policy, Lower Limit + References No definition available. + Details
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Namespace Prefix: | mcy_
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Balance Type: | debit
Period Type: | duration

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- Definition Unusual or Infrequent Item, Structures Destroyed + References No definition available. + Details
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Namespace Prefix: | mcy_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition The amount of estimated liability as of the balance sheet date for the specific conflagration, earthquake, windstorm, explosion, or similar event. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount before valuation allowance recoverable under reinsurance contracts. 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount before valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, and for amounts expected based upon statistical projections and other measures of incurred losses and loss settlement expenses which have not yet been reported to the ceding insurer. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Stated amount that is the upper retention limit per entity policy. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of risk reinsured in excess of stated retention per company policy. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Percentage of risk reinsured in excess of stated retention per entity policy. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage of risk reinsured on insurance products per entity policy. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
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Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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