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Loss And Loss Adjustment Expense Reserves (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Unusual or Infrequent Item, or Both [Line Items]
Prior year claims and claims adjustment expense	$ (15,409)	$ 27,468
Adjustments for prior year claims	539,871	612,773
Catastrophe [Member]
Unusual or Infrequent Item, or Both [Line Items]
Pre-tax catastrophe losses, net of reinsurance benefits	60,000	14,000
Liability for catastrophe claims, carrying amount	64,000	18,000
Adjustments for prior year claims	$ 4,000	$ 4,000

X

- Definition

Liability for Catastrophe Claims, Net of Reinsurance Benefits

+ References

No definition available.

+ Details

Name:	mcy_LiabilityforCatastropheClaimsNetofReinsuranceBenefits
Namespace Prefix:	mcy_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- 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 http://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

X

- Definition

Amount, after effects of reinsurance, of payments to settle claims incurred in prior periods and related claims settlement costs.

+ 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
-Subparagraph (c)
-URI http://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:	duration

X

- 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 http://asc.fasb.org/extlink&oid=116884095&loc=d3e14764-158437

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- 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

Name:	us-gaap_UnusualOrInfrequentItemLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: