General - Allowance for Credit Losses on Reinsurance Recoverables (Details) - USD ($) $ in Thousands	3 Months Ended
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

Reinsurance Recoverable, Allowance for Credit Loss, Recovery

No definition available.

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Namespace Prefix:	mcy_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Reinsurance Recoverable, Allowance for Credit Loss [Roll Forward]

+ References

No definition available.

+ Details

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Namespace Prefix:	mcy_
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Period Type:	duration

- Definition

Reinsurance Recoverable, Credit Loss Expense (Reversal)

+ References

No definition available.

+ Details

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Namespace Prefix:	mcy_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

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

- 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

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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- Details

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