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Contingencies - Loss Contingencies (Details) - California DOI [Member] - USD ($) $ in Millions |  | 1 Months Ended |
--- | --- | --- | ---
Feb. 09, 2015 | Mar. 31, 2015 | Nov. 17, 2016
Loss Contingencies [Line Items] |  |  |
Litigation, penalty assessed | $ 27.6 |  |
Payments for assessed penalty |  | $ 27.6 |
Penalty interest assessed, statutory rate |  |  | 7.00%

X
- Definition Litigation Settlement, Refund Interest Rate + References No definition available. + Details
Name: | mcy_LitigationSettlementRefundInterestRate
Namespace Prefix: | mcy_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition Amount of litigation expense, including but not limited to legal, forensic, accounting, and investigative fees. + References No definition available. + Details
Name: | us-gaap_LitigationSettlementExpense
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_LossContingenciesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of cash paid for the settlement of litigation or for other legal issues during the period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 25 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3536-108585 + Details
Name: | us-gaap_PaymentsForLegalSettlements
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Details
Name: | srt_LitigationCaseAxis=mcy_CaliforniaDOIMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |