Contingencies - Additional Information (Detail) $ in Millions									1 Months Ended		3 Months Ended	12 Months Ended
	Apr. 14, 2017	Feb. 01, 2017	Oct. 14, 2016 USD ($)	Apr. 29, 2016	Mar. 25, 2016 USD ($)	Mar. 22, 2016	Jul. 31, 2014	Dec. 26, 2012	Nov. 30, 2013 USD ($)	Dec. 31, 2012 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2012 USD ($)	Mar. 11, 2017 Customer
Loss Contingencies [Line Items]
Requested return on equity base percentage											9.20%
Disclosure of Rate Matters											On June 19, 2014, the FERC issued its decision in Complaint I, establishing an ROE methodology and setting an issue for a paper hearing. On October 16, 2014, FERC issued its final decision in Complaint I setting the base ROE at 10.57% and a maximum total ROE of 11.74% (base plus incentive ROEs)
Regulatory liabilities											$ 1,755.0	$ 1,753.0
Spent fuel litigation damages awarded, value					$ 76.8				$ 235.4				$ 160.0
Litigation appeal expiration date											Jul. 18, 2016
Spent fuel litigation damages settlement received			$ 41.6
Adjustment regulatory deferral and earning sharing accruals										$ 9.8
Staff issue settlement reserve										3.4			3.4
Customer share of earnings sharing													2.4
Price of the power purchase agreements											$ 259.0
Standby letters of credit											2,700.0
March 2017 Windstorm [Member]
Loss Contingencies [Line Items]
Number of affected customers | Customer														219,000
Connecticut Yankee Atomic Power Company [Member]
Loss Contingencies [Line Items]
Court of Federal Claims award										39.7			39.7
Spent fuel litigation damages awarded, value					32.6				126.3
Spent fuel litigation damages settlement received			18.4
Maine Yankee Atomic Power Company [Member]
Loss Contingencies [Line Items]
Court of Federal Claims award										$ 81.7			81.7
Spent fuel litigation damages awarded, value					24.6				37.7
Spent fuel litigation damages settlement received			3.6
CMP Distribution [Member]
Loss Contingencies [Line Items]
Share of the award credited back to customers									28.2		36.5
Spent fuel litigation damages settlement received												21.5
UI [Member]
Loss Contingencies [Line Items]
Share of the award credited back to customers									12.0		$ 3.8
Yankee Atomic Energy Corporation [Member]
Loss Contingencies [Line Items]
Spent fuel litigation damages awarded, value					$ 19.6				73.3
Spent fuel litigation damages settlement received			$ 19.6
Subsequent Event [Member]
Loss Contingencies [Line Items]
Midpoint of base return on equity warranted percentage	9.39%
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													38.3
Minimum [Member]
Loss Contingencies [Line Items]
Spent fuel litigation damages awarded, value									$ 82.0
Minimum [Member] | Connecticut Yankee Atomic Power Company [Member]
Loss Contingencies [Line Items]
Spent fuel litigation damages awarded, value													$ 21.4
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 Complaint I.
Regulatory liabilities											$ 21.9
Complaint II [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Approved return on equity						10.42%
Complaint III [Member]
Loss Contingencies [Line Items]
Requested return on equity base percentage							8.84%
Regulatory liabilities											$ 4.4
Complaint III [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Approved return on equity						12.19%
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 later in 2017, once FERC has enough commissioners to provide a quorum for decision-making.
Reasonably possible loss, in additional reserve, pre tax											$ 17.1
Phase III [Member] | CMP and UI [Member]
Loss Contingencies [Line Items]
Spent fuel litigation damages settlement received												$ 4.2
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		8.59%		8.61%
Unfavorable Regulatory Action [Member] | Complaint Four [Member] | Maximum [Member]
Loss Contingencies [Line Items]
Approved return on equity				11.24%
Before Amendment [Member]
Loss Contingencies [Line Items]
Approved return on equity											11.14%