Derivative Instruments and Hedging - Additional Information (Detail) - USD ($)		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) recognized in regulatory assets and liabilities				$ (21,000,000)	$ 235,000,000
Derivative financial instruments, assets		$ 119,000,000		119,000,000				$ 172,000,000
Gross Amounts of Recognized Liabilities		109,000,000		109,000,000				153,000,000
Reclassification to net income of (gains) losses on cash flow hedges, net of income taxes	[1]	(1,000,000)	$ 1,000,000	22,000,000	(25,000,000)
UI [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative financial instruments, assets		15,000,000		15,000,000				19,000,000
Regulatory Assets		73,000,000		73,000,000				75,000,000
Gross Amounts of Recognized Liabilities		89,000,000		89,000,000				95,000,000
Regulatory liabilities		0		$ 0				0
Contracts For Differences [Member] | UI [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of cost or benefit on contract allocated to customers				20.00%
Contracts For Differences [Member] | CL&P [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of cost or benefit on contract allocated to customers				80.00%
Derivative financial instruments, assets		0		$ 0				0
Gross Amounts of Recognized Liabilities		70,000,000		70,000,000				70,000,000
Network [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Net derivative losses related to discontinued cash flow hedges	[2]			(1,000,000)
Network [Member] | Cash Flow Hedging [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Net derivative losses related to discontinued cash flow hedges		2,000,000	2,000,000	4,000,000	4,000,000
Ineffective portion of Cash flow hedge		0	0	0	0
Unrealized gain(loss) from hedging activities reported on OCI				$ (1,000,000)
Derivative Instruments Gain Loss To Be Reclassified From Accumulated OCI Into Interest Expense During Next Twelve Months				12 months
Network [Member] | Cash Flow Hedging [Member] | Scenario, Forecast [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Reclassification to net income of (gains) losses on cash flow hedges, net of income taxes							$ 8,000,000
Derivative Instruments Gain Loss To Be Reclassified From Accumulated OCI Into Interest Expense During Next Twelve Months						$ 1,000,000
Network [Member] | forward starting swaps [Member] | Cash Flow Hedging [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Net loss related to previously settled forward starting swaps		72,800,000		$ 72,800,000				76,700,000
Network [Member] | Electricity Derivatives [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) recognized in regulatory assets and liabilities				(20,800,000)				(12,300,000)
Gain (Loss) reclassified from regulatory assets and liabilities into income		(10,400,000)	(21,300,000)	(13,100,000)	(47,900,000)
Network [Member] | Natural Gas Hedges [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) recognized in regulatory assets and liabilities								3,500,000
Gain (Loss) reclassified from regulatory assets and liabilities into income		0	(600,000)	0	(3,400,000)
Network [Member] | Natural Gas Hedges [Member] | Regulatory Assets [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) recognized in regulatory assets and liabilities				(600,000)
Network [Member] | Natural Gas Hedges [Member] | Regulatory Liabilities [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) recognized in regulatory assets and liabilities				$ (600,000)
Network [Member] | Fuel Derivatives [Member] | Cash Flow Hedging [Member] | Maximum [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Maximum period of time of cash flow hedges				12 months
Renewables and Gas Activities [Member] | Cash Flow Hedging [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Ineffective portion of Cash flow hedge		(800,000)	$ 400,000	$ (500,000)	$ 4,800,000
Unrealized gain(loss) from hedging activities reported on OCI				$ 2,600,000
Derivative Instruments Gain Loss To Be Reclassified From Accumulated OCI Into Interest Expense During Next Twelve Months				12 months
Net loss in accumulated OCI related to discontinued cash flow hedge				$ 500,000
Counter Party [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gross Amounts of Recognized Liabilities		21,400,000		21,400,000
Cash Collateral Pledged		21,000,000		21,000,000				$ 20,000,000
Counter Party [Member] | UI [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative collateral obligation to be paid in decrease in credit rating below investment grade		$ 11,000,000		$ 11,000,000

[1] Reclassification is reflected in the operating expenses line item in the condensed consolidated statements of income.

[2] Changes in OCI are reported on a pre-tax basis. The reclassified amounts of commodity contracts are included within “Purchase power, natural gas and fuel used” line item within operating expenses in the condensed consolidated statements of income.