Contingencies - Additional Information (Detail) - USD ($) $ in Thousands									1 Months Ended			9 Months Ended	12 Months Ended
	May 03, 2016	Apr. 29, 2016	Mar. 25, 2016	Mar. 22, 2016	Nov. 06, 2015	Mar. 05, 2015	Jul. 31, 2014	Dec. 26, 2012	Jul. 31, 2016	Nov. 30, 2013	Dec. 31, 2012	Sep. 30, 2016	Dec. 31, 2012	May 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Requested return on equity base percentage	9.55%				9.55%	10.00%						9.20%
Requested equity ratio	50.00%				50.00%	50.00%
One-time gross plant investment disallowance					$ 6,000	$ 19,950
Investment phase in amount					$ 10,000
Cash Reserve														$ 6,000	$ 6,000
New rate plan	10 years
Delivery revenue percentage	34.60%
One-time gross plant investment	$ 15,000
Disclosure of Rate Matters												On June 19, 2014, the FERC issued its decision in Complaint I, establishing a methodology and setting an issue for a paper hearing. On October 16, 2014, FERC issued its final decision in the Complaint I setting the base ROE at 10.57% and a maximum total ROE of 11.74% (base plus incentive ROEs)
Regulatory liabilities												$ 1,762,000			$ 1,841,000
Spent fuel litigation damages awarded, value			$ 76,800							$ 235,400			$ 160,000
Litigation appeal expiration date												Jul. 18, 2016
Adjustment regulatory deferral and earning sharing accruals											$ 9,800
Staff issue settlement reserve											3,400		3,400
Customer share of earnings sharing													2,400
Price of the power purchase agreements												$ 259,000
Standby letters of credit												2,700
Connecticut Yankee Atomic Power Company [Member]
Loss Contingencies [Line Items]
Court of Federal Claims award											39,700		39,700
Spent fuel litigation damages awarded, value			32,600							126,300
Maine Yankee Atomic Power Company [Member]
Loss Contingencies [Line Items]
Court of Federal Claims award											$ 81,700		81,700
Spent fuel litigation damages awarded, value			24,600							37,700
CMP Distribution [Member]
Loss Contingencies [Line Items]
Share of the award credited back to customers										28,200		36,500
UI [Member]
Loss Contingencies [Line Items]
Requested return on equity base percentage									50.00%
Share of the award credited back to customers										12,000		3,800
Yankee Atomic Energy Corporation [Member]
Loss Contingencies [Line Items]
Spent fuel litigation damages awarded, value			$ 19,600							73,300
Complaint I [Member]
Loss Contingencies [Line Items]
Regulatory liabilities												$ 21,400
Complaint II [Member]
Loss Contingencies [Line Items]
Requested return on equity base percentage								8.70%
Disclosure of Rate Matters												On December 26, 2012, a second, ROE complaint (Complaint II) for a subsequent rate period was filed requesting the ROE be reduced to 8.7%. On June 19, 2014, FERC accepted Complaint II, established a 15-month refund effective date of December 27, 2012, and set the matter for hearing using the methodology established in the Complaint I.
Regulatory liabilities												$ 21,400
Complaint III [Member]
Loss Contingencies [Line Items]
Requested return on equity base percentage							8.84%
Regulatory liabilities												$ 4,200
Complaint II and III [Member]
Loss Contingencies [Line Items]
Disclosure of Rate Matters												On November 24, 2014, FERC accepted the Complaint III, established a 15-month refund effective date of July 31, 2014, and set this matter, consolidated with Complaint II, for hearing in June 2015. Hearings were held in June 2015 on Complaints II and III before a FERC Administrative Law Judge, relating to the refund periods and going forward period. On July 29, 2015, post-hearing briefs were filed by parties and on August 26, 2015 reply briefs were filed by parties. On July 13, 2015, the NETOs filed a petition for review of FERC’s orders establishing hearing and consolidation procedures for Complaints II and III with the U.S. Court of Appeals. The FERC Administrative Law Judge issued an Initial Decision on March 22, 2016. The Initial Decision determined that, 1) for the 15-month refund period in Complaint II, the base ROE should be 9.59% and that the ROE Cap (base ROE plus incentive ROEs) should be 10.42% and 2) for the 15-month refund period in Complaint III and prospectively, the base ROE should be 10.90% and that the ROE Cap should be 12.19%. The Initial Decision is the Administrative Law Judge’s recommendation to the FERC Commissioners. The FERC is expected to make its final decision in late 2016 or early 2017.
Reasonably possible loss, in additional reserve, pre tax												$ 17,100
Reasonably possible loss, in additional reserve, net of tax												$ 10,200
Unfavorable Regulatory Action [Member] | Complaint I [Member]
Loss Contingencies [Line Items]
Approved return on equity												10.57%
Unfavorable Regulatory Action [Member] | Complaint II [Member]
Loss Contingencies [Line Items]
Approved return on equity				9.59%
Unfavorable Regulatory Action [Member] | Complaint III [Member]
Loss Contingencies [Line Items]
Approved return on equity				10.90%
Unfavorable Regulatory Action [Member] | Complaint Four [Member]
Loss Contingencies [Line Items]
Approved return on equity		8.61%
Before Amendment [Member]
Loss Contingencies [Line Items]
Approved return on equity												11.14%
Minimum [Member]
Loss Contingencies [Line Items]
Requested return on equity base percentage	14.55%
Spent fuel litigation damages awarded, value										$ 82,000
Minimum [Member] | Connecticut Yankee Atomic Power Company [Member]
Loss Contingencies [Line Items]
Spent fuel litigation damages awarded, value													21,400
Maximum [Member]
Loss Contingencies [Line Items]
Approved return on equity												11.74%
Maximum [Member] | Connecticut Yankee Atomic Power Company [Member]
Loss Contingencies [Line Items]
Spent fuel litigation damages awarded, value													$ 38,300
Maximum [Member] | Complaint II [Member]
Loss Contingencies [Line Items]
Approved return on equity				10.42%
Maximum [Member] | Complaint III [Member]
Loss Contingencies [Line Items]
Approved return on equity				12.19%
Maximum [Member] | Unfavorable Regulatory Action [Member] | Complaint Four [Member]
Loss Contingencies [Line Items]
Approved return on equity		11.24%