Other Contingencies (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Loss Contingencies Line Items
Loss Contingency, Description	E.         FERC Base ROE Complaints E.         FERC Base ROE Complaints Beginning in 2011, several New England state attorneys general, state regulatory commissions, consumer advocates, consumer groups, municipal parties and other parties (the "Complainants") jointly filed three separate complaints at FERC. In the first complaint, filed in 2011, the Complainants alleged that the NETOs' base ROE of 11.14 percent that was utilized since 2006 was unjust and unreasonable, asserted that the rate was excessive due to changes in the capital markets, and sought an order to reduce it prospectively from the date of the final FERC order and for the 15-month period beginning October 1, 2011 to December 31, 2012 (the "first complaint refund period"). In the pursuant second and third complaints, filed in 2012 and 2014, respectively, the Complainants challenged the NETOs' base ROE and sought refunds for the 15-month periods beginning December 27, 2012 and July 31, 2014, respectively. In 2014, the FERC determined that the base ROE should be set at 10.57 percent for the first complaint refund period and that a utility’s total or maximum ROE should not exceed the top of the new zone of reasonableness (7.03 percent to 11.74 percent). The FERC ordered the NETOs to provide refunds to customers for the first complaint refund period and set the new base ROE of 10.57 percent prospectively from October 16, 2014. In late 2014, the NETOs made a compliance filing, and began refunding amounts from the first complaint period, inclusive of incentive ROE adders that exceeded the 11.74 percent as compared to the total company transmission ROE. Complainants have challenged the compliance filing. As a result of the actions taken by the FERC and other developments in this matter, NU recorded reserves in 2013 and 2014 to recognize the potential financial impacts of the first and second complaints. The Company is unable to determine any amount related to the third complaint. The following is a summary of the cumulative pre-tax reserves (excluding interest) established by the Company in 2013 and 2014: As of December 31, 2014, the cumulative reserves above do not reflect refunds totaling $4.8 million at NU, $2.7 million at CL&P, $1 million at NSTAR Electric, $0.5 million at PSNH and $0.6 million at WMECO for the first complaint refund period. The aggregate after-tax net charge to 2014 earnings resulting from the 2014 FERC orders totaled $22.4 million at NU, $12.4 million at CL&P, $4.9 million at NSTAR Electric, $1.7 million at PSNH and $3.4 million at WMECO. In 2013, the aggregate after-tax charge to earnings totaled $14.3 million at NU, $7.7 million at CL&P, $3.4 million at NSTAR Electric, $1.4 million at PSNH and $1.8 million at WMECO. Although management is uncertain on the final outcome on the second and third complaints regarding the base ROE and the incentive ROE adder, management believes the current reserves established are appropriate to reflect probable and reasonably estimable refunds. F.        2014 Comprehensive Settlement Agreement On December 31, 2014, NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General filed a comprehensive settlement agreement with the DPU. The comprehensive settlement agreement included resolution of the outstanding NSTAR Electric CPSL program filings for the periods 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the NSTAR Electric energy efficiency program filings regarding LBR for the periods 2008 through 2011. If approved by the DPU, NSTAR Electric and NSTAR Gas will be required to refund a total of $44.7 million to their respective customers, which was included in regulatory liabilities as of December 31, 2014. Upon the DPU's approval, NSTAR Electric will adjust its regulatory liabilities, which it expects will result in a benefit of $23 million in the first quarter of 2015. Management expects a response from the DPU in the first quarter of 2015. G.        Basic Service Bad Debt Adder In accordance with a generic 2005 DPU order, electric utilities in Massachusetts recover the energy-related portion of bad debt costs in their Basic Service rates. In 2007, NSTAR Electric filed its 2006 Basic Service reconciliation with the DPU proposing an adjustment related to the increase of its Basic Service bad debt charge-offs. The DPU issued an order approving the implementation of a revised Basic Service rate but instructed NSTAR Electric to reduce distribution rates by an amount equal to the increase in its Basic Service bad debt charge-offs. This adjustment to NSTAR Electric's distribution rates would eliminate the fully reconciling nature of the Basic Service bad debt adder. In 2010, NSTAR Electric filed an appeal of the DPU's order with the SJC. NSTAR Electric's position was that it had fully removed the collection of energy-related bad debt costs from its distribution rates effective January 1, 2006. Therefore, no further adjustment to distribution rates was warranted. In 2012, the SJC vacated the DPU order and remanded the matter to the DPU for further review. As of December 31, 2014, NSTAR Electric has a total deferred regulatory asset of approximately $33 million of costs associated with energy-related bad debt. On January 7, 2015, the DPU issued an order on remand stating that NSTAR Electric had, in fact, removed energy-related bad debt costs from distribution rates effective January 1, 2006. The DPU order approved NSTAR Electric's 2005 and 2006 reconciliation filings and ordered NSTAR Electric and the Massachusetts Attorney General to collaborate on the submission of a proposal for the reconciliation of energy-related bad debt costs for the open years of 2007 through 2014 by April 7, 2015. Management expects to present a proposal to the Attorney General in the first quarter of 2015 with a decision from the DPU later in 2015. H.        Litigation and Legal Proceedings NU, including CL&P, NSTAR Electric, PSNH and WMECO, are involved in legal, tax and regulatory proceedings regarding matters arising in the ordinary course of business, which involve management's assessment to determine the probability of whether a loss will occur and, if probable, its best estimate of probable loss. The Company records and discloses losses when these losses are probable and reasonably estimable, and discloses matters when losses are probable but not estimable or when losses are reasonably possible. Legal costs related to the defense of loss contingencies are expensed as incurred.
FERC ROE Reserve - First Complaint - Base ROE [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	$ 24.9	$ 23.7
Loss Contingency Loss in Period	1.2
CustomerRefundLiabilityCurrent	4.8
FERC ROE Reserve - First Complaint - Base ROE [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	13.3	12.8
Loss Contingency Loss in Period	0.5
CustomerRefundLiabilityCurrent	2.7
FERC ROE Reserve - First Complaint - Base ROE [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	6.1	5.7
Loss Contingency Loss in Period	0.4
CustomerRefundLiabilityCurrent	1.0
FERC ROE Reserve - First Complaint - Base ROE [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	2.4	2.3
Loss Contingency Loss in Period	0.1
CustomerRefundLiabilityCurrent	0.5
FERC ROE Reserve - First Complaint - Base ROE [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	3.1	2.9
Loss Contingency Loss in Period	0.2
CustomerRefundLiabilityCurrent	0.6
FERC ROE Reserve - Second Complaint - Base ROE [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	27.4	0
Loss Contingency Loss in Period	27.4
FERC ROE Reserve - Second Complaint - Base ROE [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	13.5	0
Loss Contingency Loss in Period	13.5
FERC ROE Reserve - Second Complaint - Base ROE [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	7.5	0
Loss Contingency Loss in Period	7.5
FERC ROE Reserve - Second Complaint - Base ROE [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	2.7	0
Loss Contingency Loss in Period	2.7
FERC ROE Reserve - Second Complaint - Base ROE [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	3.7	0
Loss Contingency Loss in Period	3.7
FERC ROE Reserve - Incentive - First and Second Complaint [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	8.4	0
Loss Contingency Loss in Period	8.4
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	6.7	0
Loss Contingency Loss in Period	6.7
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	0	0
Loss Contingency Loss in Period	0
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	0	0
Loss Contingency Loss in Period	0
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	1.7	0
Loss Contingency Loss in Period	1.7
FERC ROE Reserve - Cumulative [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	60.7	23.7
Loss Contingency Loss in Period	37.0
FERC ROE Reserve - Cumulative [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	33.5	12.8
Loss Contingency Loss in Period	20.7
FERC ROE Reserve - Cumulative [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	13.6	5.7
Loss Contingency Loss in Period	7.9
FERC ROE Reserve - Cumulative [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	5.1	2.3
Loss Contingency Loss in Period	2.8
FERC ROE Reserve - Cumulative [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	8.5	2.9
Loss Contingency Loss in Period	5.6
FERC ROE Reserve - Cumulative - After Tax Charge [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	22.4	14.3
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	12.4	7.7
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	4.9	3.4
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	1.7	1.4
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period	$ 3.4	$ 1.8