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Employee Benefit Plans (Schedule of Restricted Stock Plan) (Details) (2008 Restricted Stock Plan [Member], USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
2008 Restricted Stock Plan [Member]
Shares
Balance	117us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	114us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	108us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Granted	33us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	28us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	21us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsGranted / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Vested	(22)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	(24)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	(15)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsExercised / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Forfeited	(1)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsForfeitures / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	(1)us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsForfeitures / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Amortization of Compensation Expense
Balance	127us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	117us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	114us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNonOptionEquityInstrumentsOutstandingNumber / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Unearned compensation
Balance	$ 1,216msex_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageGrantDateValueUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 1,039msex_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageGrantDateValueUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 1,079msex_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageGrantDateValueUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Granted	711msex_GrantedUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	589msex_GrantedUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	408msex_GrantedUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Vested
Forfeited	(10)msex_ForfeitedUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	(12)msex_ForfeitedUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Amortization of Compensation Expense	(434)msex_AmortizationOfCompensationExpenseUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	(400)msex_AmortizationOfCompensationExpenseUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	(448)msex_AmortizationOfCompensationExpenseUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Balance	$ 1,483msex_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageGrantDateValueUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 1,216msex_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageGrantDateValueUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 1,039msex_ShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsWeightedAverageGrantDateValueUnearnedCompensation / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember
Weighted Average Grant Price
Granted	$ 21.46us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 21.20us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember	$ 19.35us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue / us-gaap_AwardTypeAxis = us-gaap_RestrictedStockMember

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Section 50

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Other increase (decrease) in number of shares reserved for issuance under non-option equity instrument agreements that is not separately disclosed.

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