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Loss And Loss Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Unusual or Infrequent Item, or Both [Line Items] |  |  |
Prior years | $ 47,281 | $ (26,091) | $ 22,979
Catastrophe |  |  |
Unusual or Infrequent Item, or Both [Line Items] |  |  |
Prior years | 47,300 | (26,100) | 23,000
Pre-tax catastrophe losses, net of reinsurance benefits | 102,000 | 104,000 | 64,000
Pre-tax catastrophe losses | 101,000 | 109,000 | 69,000
Prior year claims and claim adjustment expense payments, favorable (unfavorable) outcome | $ (1,000) | $ 5,000 | $ 5,000

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- Definition Liability for Catastrophe Claims, Net of Reinsurance Benefits + References No definition available. + Details
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Namespace Prefix: | mcy_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI https://asc.fasb.org/extlink&oid=116884095&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 Sum of adjustments to the estimated reserve for unpaid claims and claims adjustment expense. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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