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Amount of obligation to return cash collateral under master netting arrangements that have not been offset against derivative assets.

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Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Derivative Instruments and Hedging - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended			12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2020	Jun. 20, 2019	May 16, 2019	Dec. 31, 2018
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Regulatory Assets	$ 2,801.0			$ 2,801.0					$ 2,945.0
Regulatory liabilities	3,542.0			3,542.0					3,428.0
UI
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gross derivative asset	2.0			2.0					5.0
Regulatory Assets	97.0			97.0					97.0
Gross amounts of recognized liabilities	99.0			99.0					102.0
Regulatory liabilities	0.0			$ 0.0					0.0
Contracts for differences \| UI
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of cost or benefit on contract allocated to customers				20.00%
Contracts for differences \| CL&P
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Percentage of cost or benefit on contract allocated to customers				80.00%
Gross derivative asset	0.0			$ 0.0					0.0
Gross amounts of recognized liabilities	96.0			96.0					96.0
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Unrealized gain (loss) from hedging activities reported in accumulated OCI				39.0
Net loss in accumulated OCI related to discontinued cash flow hedge	0.5			0.5
Amortization of net loss				2.0
Networks
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Unrealized gain (loss) from hedging activities reported in accumulated OCI	0.0	$ 0.0		1.0	$ 0.0
Networks \| Cash Flow Hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Foreign currency exchange risk							$ 100.0
Unrealized gain (loss) from hedging activities reported in accumulated OCI	(2.0)	$ (4.0)		(2.0)	$ (4.4)
Derivative instruments, losses expected to be reclassified into earnings in the next 12 months				$ 1.0
Maximum period of time of cash flow hedges				12 months
Networks \| Cash Flow Hedging \| Forecast
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative instruments, losses expected to be reclassified into earnings in the next 12 months						$ 1.0
Networks \| Fuel Derivatives \| Cash Flow Hedging \| Maximum
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Maximum period of time of cash flow hedges				12 months
Renewables and Gas Activities \| Cash Flow Hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Unrealized gain (loss) from hedging activities reported in accumulated OCI				$ (6.0)
Renewables and Gas Activities \| Cash Flow Hedging \| Forecast
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Net loss in accumulated OCI related to discontinued cash flow hedge			$ (2.0)
Counter Party
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gross amounts of recognized liabilities	10.0			10.0
Cash Collateral Pledged	12.0			12.0					26.0
Counter Party \| UI
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Derivative collateral obligation to be paid in decrease in credit rating below investment grade	12.0			12.0
Swap \| Networks \| Cash Flow Hedging
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Net loss related to previously settled forward starting swaps	$ 57.0			$ 57.0					$ 61.0
Senior Unsecured Notes Due 2029 \| Senior Notes
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Debt instrument principal amount								$ 750.0