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CONSOLIDATED STATEMENTS OF CAPITAL STOCK AND LONG-TERM DEBT (Parenthetical)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Common Stock, Shares Authorized	40,000,000us-gaap_CommonStockSharesAuthorized	40,000,000us-gaap_CommonStockSharesAuthorized
Common Stock, Shares Outstanding	16,124,000us-gaap_CommonStockSharesOutstanding	15,963,000us-gaap_CommonStockSharesOutstanding
Preferred Stock, Shares Authorized	126,000us-gaap_PreferredStockSharesAuthorized	126,000us-gaap_PreferredStockSharesAuthorized
Preferred Stock, Shares Outstanding	24,000us-gaap_PreferredStockSharesOutstanding	28,000us-gaap_PreferredStockSharesOutstanding
Amortizing Secured Note, due December 20, 2021 [Member]
Due date of debt	Dec. 20, 2021
Interest rate	8.05%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = us-gaap_SecuredDebtMember
Amortizing Secured Note, due May 19, 2028 [Member]
Due date of debt	May 19, 2028
Interest rate	6.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_SecuredDebt1Member
Amortizing Secured Note, due August 25, 2030 [Member]
Due date of debt	Aug. 25, 2030
Interest rate	6.44%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_SecuredDebt2Member
Amortizing Secured Note, due September 19, 2031 [Member]
Due date of debt	Sep. 19, 2031
Interest rate	6.46%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_SecuredDebt3Member
State Revolving Trust Note, due December 31, 2022 [Member]
Due date of debt	Dec. 31, 2022
Interest rate	4.22%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustNoteMember
State Revolving Trust Note, due January 25, 2027 [Member]
Due date of debt	Jan. 25, 2027
Interest rate	3.49%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustNote2Member
State Revolving Trust Note, due May 1, 2025 [Member]
Due date of debt	May 01, 2025
Interest rate	3.60%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustNote1Member
State Revolving Trust Note, due March 1, 2026 [Member]
Due date of debt	Mar. 01, 2026
Interest rate	3.30%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustNote9Member
State Revolving Trust Note, due December 1, 2026 [Member]
Due date of debt	Dec. 01, 2026
Interest rate	4.03%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustNote3Member
State Revolving Trust Bond 4.00% to 5.00%, due August 1, 2021 [Member]
Due date of debt	Aug. 01, 2021
Interest rate - minimum	4.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustBondMember
Interest rate - maximum	5.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustBondMember
State Revolving Trust Bond 0.00%, due August 1, 2021 [Member]
Due date of debt	Aug. 01, 2021
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustBond1Member
State Revolving Trust Note, due July 1, 2028 [Member]
Due date of debt	Jul. 01, 2028
Interest rate	3.64%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustNote4Member
State Revolving Trust Note, due January 1, 2028 [Member]
Due date of debt	Jan. 01, 2028
Interest rate	3.64%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustNote5Member
State Revolving Trust Note, due August 1, 2031 [Member]
Due date of debt	Aug. 01, 2031
Interest rate	3.45%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustNote6Member
Amortizing Secured Note, due April 20, 2029 [Member]
Due date of debt	Apr. 20, 2029
Interest rate	6.59%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_SecuredDebt4Member
Amortizing Secured Note 7.05%, due January 20, 2030 [Member]
Due date of debt	Jan. 20, 2030
Interest rate	7.05%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_SecuredDebt5Member
Amortizing Secured Note 5.69%, due January 20, 2030 [Member]
Due date of debt	Jan. 20, 2030
Interest rate	5.69%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_SecuredDebt6Member
Amortizing Secured Note, due April 20, 2040 [Member]
Due date of debt	Apr. 20, 2040
State Revolving Trust Note, due July 1, 2031 [Member]
Due date of debt	Jul. 01, 2031
Interest rate	3.75%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustNote7Member
State Revolving Trust Note, due November 30, 2030 [Member]
Due date of debt	Nov. 30, 2030
Interest rate	3.75%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_StateRevolvingTrustNote8Member
First Mortgage Bonds - Series X [Member]
Due date of debt	Sep. 01, 2018
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage1Member
First Mortgage Bonds - Series Y [Member]
Due date of debt	Sep. 01, 2018
Interest rate - minimum	4.25%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage2Member
Interest rate - maximum	4.63%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage2Member
First Mortgage Bonds - Series Z [Member]
Due date of debt	Sep. 01, 2019
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage3Member
First Mortgage Bonds - Series AA [Member]
Due date of debt	Sep. 01, 2019
Interest rate - minimum	5.25%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage4Member
Interest rate - maximum	5.75%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage4Member
First Mortgage Bonds - Series BB [Member]
Due date of debt	Sep. 01, 2021
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage5Member
First Mortgage Bonds - Series CC [Member]
Due date of debt	Sep. 01, 2021
Interest rate - minimum	4.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage6Member
Interest rate - maximum	5.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage6Member
First Mortgage Bonds - Series EE [Member]
Due date of debt	Aug. 01, 2023
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage8Member
First Mortgage Bonds - Series FF [Member]
Due date of debt	Aug. 01, 2024
Interest rate - minimum	3.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage9Member
Interest rate - maximum	5.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage9Member
First Mortgage Bonds - Series GG [Member]
Due date of debt	Aug. 01, 2026
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage10Member
First Mortgage Bonds - Series HH [Member]
Due date of debt	Aug. 01, 2026
Interest rate - minimum	4.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage11Member
Interest rate - maximum	5.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage11Member
First Mortgage Bonds - Series II [Member]
Due date of debt	Aug. 01, 2024
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage12Member
First Mortgage Bonds - Series JJ [Member]
Due date of debt	Aug. 01, 2027
Interest rate - minimum	3.40%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage13Member
Interest rate - maximum	5.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage13Member
First Mortgage Bonds - Series KK [Member]
Due date of debt	Aug. 01, 2028
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage14Member
First Mortgage Bonds - Series LL [Member]
Due date of debt	Aug. 01, 2028
Interest rate - minimum	5.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage15Member
Interest rate - maximum	5.50%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage15Member
First Mortgage Bonds - Series MM [Member]
Due date of debt	Aug. 01, 2030
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage16Member
First Mortgage Bonds - Series NN [Member]
Due date of debt	Aug. 01, 2030
Interest rate - minimum	3.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage17Member
Interest rate - maximum	4.375%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage17Member
First Mortgage Bonds - Series OO [Member]
Due date of debt	Aug. 01, 2031
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage18Member
First Mortgage Bonds - Series PP [Member]
Due date of debt	Aug. 01, 2031
Interest rate - minimum	2.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage19Member
Interest rate - maximum	5.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage19Member
First Mortgage Bonds - Series QQ [Member]
Due date of debt	Oct. 01, 2023
Interest rate	5.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage20Member
First Mortgage Bonds - Series RR [Member]
Due date of debt	Oct. 01, 2038
Interest rate	3.80%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage21Member
First Mortgage Bonds - Series SS [Member]
Due date of debt	Oct. 01, 2047
Interest rate	4.25%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage22Member
First Mortgage Bonds - Series TT [Member]
Due date of debt	Aug. 01, 2032
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage23Member
First Mortgage Bonds - Series UU [Member]
Due date of debt	Aug. 01, 2032
Interest rate - minimum	3.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage24Member
Interest rate - maximum	3.25%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage24Member
First Mortgage Bonds - Series VV [Member]
Due date of debt	Aug. 01, 2033
Interest rate	0.00%us-gaap_DebtInstrumentInterestRateStatedPercentage / us-gaap_DebtInstrumentAxis = msex_FirstMortgage25Member
First Mortgage Bonds - Series WW [Member]
Due date of debt	Aug. 01, 2033
Interest rate - minimum	3.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMinimum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage26Member
Interest rate - maximum	5.00%us-gaap_DebtInstrumentInterestRateStatedPercentageRateRangeMaximum / us-gaap_DebtInstrumentAxis = msex_FirstMortgage26Member
Convertible Preferred Stock $7.00 Series
Preferred Stock, Shares Outstanding	10,000us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_ConvertiblePreferredStockMember	14,000us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_ConvertiblePreferredStockMember
Convertible Preferred Stock $8.00 Series [Member]
Preferred Stock, Shares Outstanding	3,000us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = msex_ConvertiblePreferredStock1Member	3,000us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = msex_ConvertiblePreferredStock1Member
Nonredeemable Preferred Stock $7.00 Series
Preferred Stock, Shares Outstanding	1,000us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_NonredeemablePreferredStockMember	1,000us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = us-gaap_NonredeemablePreferredStockMember
Nonredeemable Preferred Stock $4.75 Series [Member]
Preferred Stock, Shares Outstanding	10,000us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = msex_NonredeemablePreferredStock1Member	10,000us-gaap_PreferredStockSharesOutstanding / us-gaap_StatementClassOfStockAxis = msex_NonredeemablePreferredStock1Member

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