CAPITALIZATION (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended					1 Months Ended
	Oct. 31, 2013 Subsequent Event [Member]	Oct. 25, 2013 Subsequent Event [Member] United Illuminating Company (UI) [Member] Senior Unsecured Notes Due 2023 [Member]	Oct. 25, 2013 Subsequent Event [Member] Connecticut Natural Gas Corporation (CNG) [Member] Senior Unsecured Notes Due 2028 [Member]	Oct. 25, 2013 Subsequent Event [Member] Connecticut Natural Gas Corporation (CNG) [Member] Senior Unsecured Notes Due 2043 [Member]	Oct. 25, 2013 Subsequent Event [Member] Connecticut Natural Gas Corporation (CNG) [Member] Series D Medium Term Notes Due 2013 [Member]	Oct. 25, 2013 Subsequent Event [Member] Berkshire [Member] Senior Unsecured Notes Due 2043 [Member]	Sep. 30, 2013 Common Stock [Member]
Temporary Equity [Line Items]
Common stock outstanding (in shares)							50,712,507
Common stock par value (in dollars per share)							$ 0
Common stock issued in underwritten public offering (in shares)	5,750,000
Common stock issued (in dollars per share)	$ 37.25
Shares issued upon exercise of underwriters over-allotment option (in shares)	750,000
Net proceeds of the offering	$ 206.4
Long-term debt [Abstract]
Interest rate (in hundredths)		3.95%	4.30%	5.23%	6.50%	5.33%
Long-term debt payable		75	25	20	20	15
Debt instrument, maturity date		Oct. 25, 2023	Oct. 25, 2028	Oct. 25, 2043		Dec. 10, 2043
Proceeds from Long-term debt issuance used to replenish cash			20
Proceeds from long-term debt issuance to be used for future debt repayment				$ 20