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Employee Benefit Plans (Details 4) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2012
Pension Benefit Plan |
Expected amortized actuarial Loss | $ 1,632
Expected amortized Prior Service Cost | 10
Other Benefits Plan |
Expected amortized actuarial Loss | 1,976
Expected amortized Prior Service Cost |

X
- Definition The amount of actuarial loss expected to be amortized from Regulatory Assets into net Periodic Benefit Cost. + References No definition available. + Details
Name: | MSEX_ExpectedAmortizedActuarialLoss
Namespace Prefix: | MSEX_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition The amount of Prior Service Cost expected to be amortized from Regulatory Assets into net Periodic Benefit Cost. + References No definition available. + Details
Name: | MSEX_ExpectedAmortizedPriorServiceCost
Namespace Prefix: | MSEX_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration