Derivative Instruments and Hedging - Additional Information (Detail) - USD ($)		3 Months Ended		12 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) recognized in regulatory assets and liabilities		$ 45,000,000	$ 67,000,000
Derivative financial instruments, assets		133,000,000				$ 172,000,000
Gross Amounts of Recognized Liabilities		130,000,000				153,000,000
Reclassification to net income of (gains) losses on cash flow hedges, net of income taxes	[1]	23,000,000	(26,000,000)
UI [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative financial instruments, assets		17,000,000				19,000,000
Regulatory Assets		81,000,000				75,000,000
Gross Amounts of Recognized Liabilities		98,000,000				95,000,000
Regulatory liabilities		$ 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
Gross Amounts of Recognized Liabilities		76,000,000				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
Ineffective portion of Cash flow hedge		0	0
Unrealized gain(loss) from hedging activities reported on OCI		$ (800,000)
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				$ 800,000
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		$ 74,900,000				76,700,000
Network [Member] | Electricity Derivatives [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) recognized in regulatory assets and liabilities		(20,100,000)				(12,300,000)
Gain (Loss) reclassified from regulatory assets and liabilities into income		(10,900,000)	(34,800,000)
Network [Member] | Natural Gas Hedges [Member]
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gain (Loss) recognized in regulatory assets and liabilities		500,000				3,500,000
Gain (Loss) reclassified from regulatory assets and liabilities into income		$ 600,000	(3,400,000)
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		$ (300,000)	$ (4,400,000)
Unrealized gain(loss) from hedging activities reported on OCI		$ 500,000
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		20,100,000
Cash Collateral Pledged		23,000,000				$ 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

[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.