Contingencies (Detail) $ in Millions							1 Months Ended	3 Months Ended
	Feb. 21, 2019 USD ($)	May 18, 2018 USD ($)	Apr. 30, 2018	Mar. 22, 2016	Mar. 03, 2015	Oct. 16, 2014	Oct. 31, 2018	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2018 Customer Storm	Mar. 11, 2017 Customer
Loss Contingencies [Line Items]
Regulatory liabilities								$ 3,253.0	$ 3,223.0
Price of the power purchase agreements								259.0
Requested renewables delay from preliminary proposed ruling period			2 years
Standby letters of credit								476.0
March 2017 Windstorm
Loss Contingencies [Line Items]
Number of affected customers | Customer											219,000
Investments to increase resiliency and improve emergency response in the areas impacted by the storm		$ 3.9
March 2018 Windstorm
Loss Contingencies [Line Items]
Number of affected customers | Customer										520,000
Number of severe winter storm | Storm										2
Complaint II
Loss Contingencies [Line Items]
Regulatory liabilities								23.7
Complaint III
Loss Contingencies [Line Items]
Regulatory liabilities								6.4
Complaint II and III
Loss Contingencies [Line Items]
Reasonably possible loss, in additional reserve, pre tax								$ 17.1
Unfavorable Regulatory Action | Complaint I
Loss Contingencies [Line Items]
Approved return on equity				9.59%		10.57%	9.60%
Requested existing base return on equity base percentage							10.41%
Unfavorable Regulatory Action | Complaint I | Maximum
Loss Contingencies [Line Items]
Approved return on equity				10.42%	11.74%	11.74%	10.99%
Requested existing base return on equity base percentage							13.08%
Unfavorable Regulatory Action | Complaint III
Loss Contingencies [Line Items]
Approved return on equity				10.90%
Unfavorable Regulatory Action | Complaint III | Maximum
Loss Contingencies [Line Items]
Approved return on equity				12.19%
Before Amendment
Loss Contingencies [Line Items]
Approved return on equity								11.14%
Yankee Companies
Loss Contingencies [Line Items]
Amount awarded to other party	$ 103.2
Connecticut Yankee
Loss Contingencies [Line Items]
Amount awarded to other party	40.7
Maine Yankee
Loss Contingencies [Line Items]
Amount awarded to other party	34.4
Yankee Atomic
Loss Contingencies [Line Items]
Amount awarded to other party	$ 28.1