Contingencies (Detail) customer in Thousands, $ in Millions						1 Months Ended		3 Months Ended
	Feb. 06, 2020 USD ($)	Apr. 18, 2019 recommendation	Mar. 22, 2016	Mar. 03, 2015	Oct. 16, 2014	Oct. 31, 2018 complaint	Apr. 30, 2018	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	May 01, 2018 USD ($)	Mar. 31, 2018 storm customer
Loss Contingencies [Line Items]
Regulatory liabilities								$ 3,301.0	$ 3,281.0
Price of the power purchase agreements								259.0
Requested renewables delay from preliminary proposed ruling period							2 years
Standby letters of credit								441.0
March 2018 Windstorm
Loss Contingencies [Line Items]
Number of severe winter storm | storm											2
Number of affected customers | customer											520
Amount awarded to other party	$ 10.5
Complaint II
Loss Contingencies [Line Items]
Regulatory liabilities								25.0
Complaint III
Loss Contingencies [Line Items]
Regulatory liabilities								7.0
Complaint II and III
Loss Contingencies [Line Items]
Reasonably possible loss, in additional reserve, pre tax								$ 17.0
Unfavorable Regulatory Action | Complaint I
Loss Contingencies [Line Items]
Approved return on equity			9.59%		10.57%	9.88%
Refund period term								15 months
Requested existing base return on equity base percentage						10.41%
Number of claims | complaint						4
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 I | Minimum
Loss Contingencies [Line Items]
Approved return on equity						9.60%
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%
Amphora Gas Storage USA, LLC
Loss Contingencies [Line Items]
Claim for indemnification										$ 20.0
Indemnification liability, percentage of purchase price										15.00%
Indemnification liability, maximum amount										$ 10.0
NYPSC
Loss Contingencies [Line Items]
Number of recommendations | recommendation		94