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General - Allowance for Credit Losses on Premium Receivables (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2020 | Mar. 31, 2019
Premiums Receivable, Allowance for Doubtful Accounts [Roll Forward] |  |
Beginning balance | $ 1,445 | $ 1,458
Provision during the period for expected credit losses | 8,169 | 1,563
Write-off amounts during the period | (1,658) | (1,870)
Recoveries during the period of amounts previously written off | 189 | 170
Ending balance | 10,000 | 1,321
Cumulative Effect, Period Of Adoption, Adjustment [Member] |  |
Premiums Receivable, Allowance for Doubtful Accounts [Roll Forward] |  |
Beginning balance | 1,855 | 0
Cumulative Effect, Period Of Adoption, Adjusted Balance [Member] |  |
Premiums Receivable, Allowance for Doubtful Accounts [Roll Forward] |  |
Beginning balance | $ 3,300 | $ 1,458

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- Definition Premium Receivable, Allowance for Credit Loss, Writeoff + References No definition available. + Details
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Namespace Prefix: | mcy_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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

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- Definition The carrying value as of the balance sheet date of the estimated valuation allowance to reduce gross premiums receivable to net realizable value. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(5)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524 + Details
Name: | us-gaap_PremiumsReceivableAllowanceForDoubtfulAccounts
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of recoveries of premiums receivable that were previously written off as bad debts. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524 + Details
Name: | us-gaap_PremiumsReceivableAllowanceForDoubtfulAccountsRecoveries
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |

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- Details
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Balance Type: |
Period Type: |