Regulatory Assets and Liabilities - PURA Investigation of the Preparation for and Response to the Tropical Storm Isaias and Connecticut Storm Reimbursement Legislation (Details) - USD ($) $ in Millions	Jul. 14, 2021	May 06, 2021	Apr. 15, 2021
Public Utilities, General Disclosures [Line Items]
Civil penalty amount	$ 1	$ 2
United Illuminating Company (UI)
Public Utilities, General Disclosures [Line Items]
ROE reduction			0.15%