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Commitments and Contingencies (Narrative) (Details) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Apr. 01, 2021 | Dec. 31, 2020
Commitments and Contingencies Disclosure [Abstract] |  |
Exposure to earthquake loss | $ 77.1 |
Reduction in premiums written, net |  | $ 128.0

X
- Definition Catastrophe Claims, Maximum Exposure, Carrying Amount + References No definition available. + Details
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Namespace Prefix: | mcy_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of decrease (increase) in unearned premiums. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration