Other Contingencies (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2015
Loss Contingencies Line Items
Public Utilities Disclosure Of Regulatory Matters	2015 Regulatory Developments: FERC ROE Complaints: As a result of the March 3, 2015 FERC order in the pending ROE complaint proceedings described in Note 8C, "Commitments and Contingencies – FERC ROE Complaints," in 2015, Eversource recognized a pre-tax charge to earnings (excluding interest) of $20 million, of which $12.5 million was recorded at CL&P, $2.4 million at NSTAR Electric, $1 million at PSNH, and $4.1 million at WMECO. The pre-tax charge was recorded as a regulatory liability and as a reduction to Operating Revenues. NSTAR Electric and NSTAR Gas 2014 Comprehensive Settlement Agreement: On March 2, 2015, the DPU approved the comprehensive settlement agreement between NSTAR Electric, NSTAR Gas and the Massachusetts Attorney General (the "Settlement") as filed with the DPU on December 31, 2014. The Settlement resolved the outstanding NSTAR Electric CPSL program filings for 2006 through 2011, the NSTAR Electric and NSTAR Gas PAM and energy efficiency-related customer billing adjustments reported in 2012, and the recovery of LBR related to NSTAR Electric's energy efficiency programs for 2009 through 2011 (11 dockets in total). In 2015, as a result of the DPU order, NSTAR Electric and NSTAR Gas commenced refunding a combined $44.7 million to customers, which was recorded as a regulatory liability. NSTAR Electric recognized a $21.7 million pre-tax benefit in the first quarter of 2015 as a result of the approval of the Settlement. NSTAR Electric Basic Service Bad Debt Adder: On January 7, 2015, the DPU issued an order concluding that NSTAR Electric had removed energy-related bad debt costs from base distribution rates effective January 1, 2006. As a result of the DPU order, NSTAR Electric increased its regulatory assets and reduced operations and maintenance expense by $24.2 million in the first quarter of 2015, resulting in after-tax earnings of $14.5 million. On May 5, 2015, NSTAR Electric filed for recovery of the energy-related bad debt costs regulatory asset from customers beginning July 1, 2015. On June 24, 2015, the DPU delayed the effective date of NSTAR Electric’s proposed rate increase from July 1, 2015 to November 1, 2015 to allow for the DPU staff to review the reconciliations. NSTAR Electric requested recovery from customers effective January 1, 2016 in briefs filed with the DPU in October 2015. On October 27, 2015, the DPU delayed the effective date of the rate increase from November 1, 2015 to December 1, 2015 to allow the DPU staff additional time to review the reconciliations. NSTAR Electric expects a decision from the DPU in the fourth quarter of 2015. CL&P Distribution Rates: On July 2, 2015, PURA issued a final order that allows for an increase to rate base of approximately $163 million associated with ADIT, including a regulatory asset to recover the incremental revenue requirement for the period December 1, 2014 through November 30, 2015 over a subsequent two-year period. The rate base increase provided an increase to total allowed annual revenue requirements of $18.4 million beginning December 1, 2014. Of that amount, $15.3 million has been recorded as a regulatory asset through September 30, 2015, with a corresponding increase in Operating Revenues.
FERC ROE Reserve - Incentive - First and Second Complaint [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period		$ 20.0
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period		12.5
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period		2.4
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period		1.0
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Loss in Period		$ 4.1