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Commitments and Contingent Liabilities (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended |
--- | --- | ---
Dec. 31, 2014 | Dec. 31, 2013
Commitment And Contingencies [Line Items] |  |
Period of wastewater agreement | 20 years |
Term of water and wastewater system agreement | USA-PA operates the City of Perth Amboy, NJ's water and wastewater systems under a 20-year agreement, which expires in 2018. In connection with the agreement with Perth Amboy, USA-PA entered into a 20-year subcontract with a wastewater operating company |
Guarant liabilty for AWM's performance | $ 0.3 us-gaap_GuarantyLiabilities | $ 0.4 us-gaap_GuarantyLiabilities
Budgeted construction cost for construction program, 2015 | 28.6 msex_BudgetedConstructionCost |
Budgeted construction cost for construction program, 2016 | 33.9 msex_BudgetedConstructionCostForConstructionProgramYearOne |
Budgeted construction cost for construction program, 2017 | $ 33.1 msex_BudgetedConstructionCostForConstructionProgramYearTwo |
NJ Water Supply Authority [Member] |  |
Commitment And Contingencies [Line Items] |  |
Commitment of purchase of untreated water | Middlesex has an agreement with the NJWSA for the purchase of untreated water through November 30, 2023, which provides for an average purchase of 27 million gallons a day (mgd). Pricing is set annually by the NJWSA through a public rate making process. |
Purchase commitment expiration date of contract | Nov. 30, 2023 |
Water purchase per commitment | 27,000,000 us-gaap_LongTermPurchaseCommitmentMinimumQuantityRequired / us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis = us-gaap_PurchaseCommitmentMember |
Regulated Water Authority [Member] |  |
Commitment And Contingencies [Line Items] |  |
Minimum treated water purchase commitment | Middlesex also has an agreement with a non-affiliated regulated water utility for the purchase of treated water. This agreement, which expires February 27, 2016, provides for the minimum purchase of 3 mgd of treated water with provisions for additional purchases. |
Purchase commitment expiration date of contract | Feb. 27, 2016 |
Water purchase per commitment | 3,000,000 us-gaap_LongTermPurchaseCommitmentMinimumQuantityRequired / us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis = msex_PurchaseCommitment1Member |
City of Dover [Member] |  |
Commitment And Contingencies [Line Items] |  |
Water purchase per commitment | 15,000,000 us-gaap_LongTermPurchaseCommitmentMinimumQuantityRequired / us-gaap_PurchaseCommitmentExcludingLongtermCommitmentAxis = msex_PurchaseCommitment2Member |

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- Definition This item represents a non-contingent liability for the fair value of an obligation to stand ready to perform over the term of a guaranty issued in the event that specified triggering events or conditions occur. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.15) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.15) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 25 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=7661617&loc=d3e11049-110243 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 25 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=7661617&loc=d3e11019-110243 + Details
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