Regulatory Assets and Liabilities - Additional Information (Detail)				1 Months Ended	3 Months Ended		9 Months Ended
	May 03, 2016	Nov. 06, 2015	Mar. 05, 2015	Jul. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2016 USD ($) Station	Dec. 31, 2015 USD ($)
Regulatory Assets And Liabilities [Line Items]
Unrecorded Regulatory Assets					$ 2,700,000,000		$ 2,700,000,000
Unfunded future income tax expense						$ 126,000,000	$ 126,000,000
Unfunded future Income tax expense collection period							50 years
Percentage of earnings in excess of allowed ROE	9.55%	9.55%	10.00%				9.20%
Number of nuclear generating stations | Station							2
UIL Holdings [Member]
Regulatory Assets And Liabilities [Line Items]
Business combination merger related rate credits					$ 0		$ 20,000,000
Minimum [Member]
Regulatory Assets And Liabilities [Line Items]
Percentage of earnings in excess of allowed ROE	14.55%
Maximum [Member]
Regulatory Assets And Liabilities [Line Items]
Percentage of return on equity							11.74%
NEW YORK
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years
NEW YORK | Prior Tax Rate [Member]
Regulatory Assets And Liabilities [Line Items]
Statutory income tax rate, state							7.10%
NEW YORK | Revised Tax Rate [Member]
Regulatory Assets And Liabilities [Line Items]
Statutory income tax rate, state							6.50%
NEW YORK | Minimum [Member]
Regulatory Assets And Liabilities [Line Items]
Deferred income tax recovery period							27 years
NEW YORK | Maximum [Member]
Regulatory Assets And Liabilities [Line Items]
Deferred income tax recovery period							39 years
Deferred Property Tax [Member] | NEW YORK
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years
Asset Sale Gain Account [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years
Carrying Costs On Deferred Income Tax Bonus Depreciation [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years
Economic Development [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years
Merger Capital Expense [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years
Positive Benefit Adjustment [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years
Post Term Amortization [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years
Theoretical Reserve Flow Thru Impact [Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years
Electric and Gas Service Rate Plan Year One [Member]
Regulatory Assets And Liabilities [Line Items]
Increase in customer earning levels							50.00%
Percentage of return on equity							9.50%
Electric and Gas Service Rate Plan Year Two [Member]
Regulatory Assets And Liabilities [Line Items]
Increase in customer earning levels							75.00%
Percentage of return on equity							10.00%
Electric and Gas Service Rate Plan Year Three [Member]
Regulatory Assets And Liabilities [Line Items]
Increase in customer earning levels							90.00%
Percentage of return on equity							10.50%
NYSEG [Member]
Regulatory Assets And Liabilities [Line Items]
Annual amortization of regulatory items							$ 16,500,000
Regulatory items amortization period							3 years
Approved return on equity							9.00%
Equity ratio					48.00%		48.00%
Equity ratio for earnings sharing					50.00%		50.00%
NYSEG [Member] | Storm Costs
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years
NYSEG [Member] | Regulatory Items Other Than Storm Costs [ Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							10 years
NYSEG [Member] | Deferred Income Tax Charge
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							15 years
RGE Electric [Member]
Regulatory Assets And Liabilities [Line Items]
Approved return on equity							9.00%
Equity ratio					48.00%		48.00%
Equity ratio for earnings sharing					50.00%		50.00%
UI [Member]
Regulatory Assets And Liabilities [Line Items]
Contract for purchase of electric power, description							On July 1, 2016, the United Illuminating Company (UI) filed an application with the Connecticut Public Utilities Regulatory Authority (PURA) requesting approval of a three-year rate plan commencing January 1, 2017, and extending through December 31, 2019
Contract for purchase of electric power, expiration date				Dec. 31, 2019
Requested revenue increase amount				$ 100,100,000
Modified requested revenue increase amount				98,300,000
Aggregate value of three year rate plan levelized recovery to nonlevelized revenue increase				$ 25,600,000
Percentage of earnings in excess of allowed ROE				50.00%
Regulatory assets					$ 84,000,000		$ 84,000,000	$ 68,000,000
UI [Member] | Electric and Gas Service Rate Plan Year One [Member]
Regulatory Assets And Liabilities [Line Items]
Requested revenue increase amount				$ 65,600,000
Levelized revenue increase amount				$ 40,700,000
Proposed deadband under earnings sharing mechanism (ESM)				0.20%
UI [Member] | Electric and Gas Service Rate Plan Year Two [Member]
Regulatory Assets And Liabilities [Line Items]
Requested revenue increase amount				$ 21,100,000
Levelized revenue increase amount				$ 47,400,000
Proposed deadband under earnings sharing mechanism (ESM)				0.30%
UI [Member] | Electric and Gas Service Rate Plan Year Three [Member]
Regulatory Assets And Liabilities [Line Items]
Requested revenue increase amount				$ 13,400,000
Levelized revenue increase amount				$ 39,100,000
Proposed deadband under earnings sharing mechanism (ESM)				0.40%
UI [Member] | Storm Costs
Regulatory Assets And Liabilities [Line Items]
Deferred costs					1,000,000		1,000,000
Regulatory assets					$ 0		0
Central Maine Power Company [Member]
Regulatory Assets And Liabilities [Line Items]
Recovery of deferred storm costs							$ 123,000,000
Central Maine Power Company [Member] | Storm Costs
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							10 years
Central Maine Power Company [Member] | Regulatory Items Other Than Storm Costs [ Member]
Regulatory Assets And Liabilities [Line Items]
Regulatory items amortization period							5 years