Regulatory Assets and Liabilities - Rate Increases and Delivery Rate Percentages (Details) $ in Thousands	Jun. 22, 2020 USD ($)
NYSEG Electric | Electric and Gas Service Rate Plan Year One
Regulatory Liabilities [Line Items]
Rate Increase	$ 34,700
Delivery Rate Increase	4.60%
NYSEG Electric | Electric and Gas Service Rate Plan Year Two
Regulatory Liabilities [Line Items]
Rate Increase	$ 71,510
Delivery Rate Increase	9.10%
NYSEG Electric | Electric and Gas Service Rate Plan Year Three
Regulatory Liabilities [Line Items]
Rate Increase	$ 79,400
Delivery Rate Increase	9.10%
NYSEG Gas | Electric and Gas Service Rate Plan Year One
Regulatory Liabilities [Line Items]
Rate Increase	$ 0
Delivery Rate Increase	0.00%
NYSEG Gas | Electric and Gas Service Rate Plan Year Two
Regulatory Liabilities [Line Items]
Rate Increase	$ 1,580
Delivery Rate Increase	0.80%
NYSEG Gas | Electric and Gas Service Rate Plan Year Three
Regulatory Liabilities [Line Items]
Rate Increase	$ 3,300
Delivery Rate Increase	1.60%
RG&E Electric | Electric and Gas Service Rate Plan Year One
Regulatory Liabilities [Line Items]
Rate Increase	$ 10,700
Delivery Rate Increase	2.40%
RG&E Electric | Electric and Gas Service Rate Plan Year Two
Regulatory Liabilities [Line Items]
Rate Increase	$ 22,920
Delivery Rate Increase	5.20%
RG&E Electric | Electric and Gas Service Rate Plan Year Three
Regulatory Liabilities [Line Items]
Rate Increase	$ 25,400
Delivery Rate Increase	5.20%
RG&E Gas | Electric and Gas Service Rate Plan Year One
Regulatory Liabilities [Line Items]
Rate Increase	$ 0
Delivery Rate Increase	0.00%
RG&E Gas | Electric and Gas Service Rate Plan Year Two
Regulatory Liabilities [Line Items]
Rate Increase	$ 0
Delivery Rate Increase	0.00%
RG&E Gas | Electric and Gas Service Rate Plan Year Three
Regulatory Liabilities [Line Items]
Rate Increase	$ 2,400
Delivery Rate Increase	1.30%