PREFFERED STOCK NOT SUBJECT TO MANDATORY REDEMPTION (DETAILS) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Class Of Stock [Line Items]
Preferred Stock, Value, Outstanding	$ 155,600,000	$ 155,600,000
Cash Dividends on Preferred Stock	7,519,000	7,682,000	7,029,000
The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	2,324,000
Preferred Stock, Value, Outstanding	116,200,000	116,200,000
Cash Dividends on Preferred Stock	5,559,000	5,559,000	5,559,000
Preferred Stock Authorized Description	The CL&P and NSTAR Electric preferred stock is not subject to mandatory redemption and is presented as a noncontrolling interest of a subsidiary in NU’s financial statements. CL&P Preferred Stock: CL&P's charter authorizes it to issue up to 9 million shares of preferred stock ($50 par value per share).
NSTAR Electric Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	430,000
Preferred Stock, Value, Outstanding	43,000,000	43,000,000
Cash Dividends on Preferred Stock	1,961,000	2,123,000	1,960,000
Preferred Stock Authorized Description	NSTAR Electric is authorized to issue 2,890,000 shares ($100 par value per share).
Series 1947 $1.90 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	163,912
Preferred Stock, Redemption Price Per Share	$ 52.50
Preferred Stock, Value, Outstanding	8,200,000	8,200,000
Series 1947 $2.00 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	336,088
Preferred Stock, Redemption Price Per Share	$ 54.00
Preferred Stock, Value, Outstanding	16,800,000	16,800,000
Series 1949 $2.04 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	100,000
Preferred Stock, Redemption Price Per Share	$ 52.00
Preferred Stock, Value, Outstanding	5,000,000	5,000,000
Series 1949 $2.20 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	200,000
Preferred Stock, Redemption Price Per Share	$ 52.50
Preferred Stock, Value, Outstanding	10,000,000	10,000,000
Series 1949 3.90% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	160,000
Preferred Stock, Redemption Price Per Share	$ 50.50
Preferred Stock, Value, Outstanding	8,000,000	8,000,000
Series E 1954 $2.06 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	200,000
Preferred Stock, Redemption Price Per Share	$ 51.00
Preferred Stock, Value, Outstanding	10,000,000	10,000,000
Series F 1955 $2.09 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	100,000
Preferred Stock, Redemption Price Per Share	$ 51.00
Preferred Stock, Value, Outstanding	5,000,000	5,000,000
Series 1956 4.5% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	104,000
Preferred Stock, Redemption Price Per Share	$ 50.75
Preferred Stock, Value, Outstanding	5,200,000	5,200,000
Series 1958 4.96% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	100,000
Preferred Stock, Redemption Price Per Share	$ 50.50
Preferred Stock, Value, Outstanding	5,000,000	5,000,000
Series 1963 4.5% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	160,000
Preferred Stock, Redemption Price Per Share	$ 50.50
Preferred Stock, Value, Outstanding	8,000,000	8,000,000
Series 1967 5.28% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	200,000
Preferred Stock, Redemption Price Per Share	$ 51.43
Preferred Stock, Value, Outstanding	10,000,000	10,000,000
Series G of 1968 $3.24 [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	300,000
Preferred Stock, Redemption Price Per Share	$ 51.84
Preferred Stock, Value, Outstanding	15,000,000	15,000,000
Series 1968 6.56% [Member] | The Connecticut Light And Power Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	200,000
Preferred Stock, Redemption Price Per Share	$ 51.44
Preferred Stock, Value, Outstanding	10,000,000	10,000,000
Preferred Stock 4.25% [Member] | NSTAR Electric Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	180,000
Preferred Stock, Redemption Price Per Share	$ 103.625
Preferred Stock, Value, Outstanding	18,000,000	18,000,000
Preferred Stock 4.78% [Member] | NSTAR Electric Company [Member]
Class Of Stock [Line Items]
Preferred Stock, Shares Outstanding	250,000
Preferred Stock, Redemption Price Per Share	$ 102.80
Preferred Stock, Value, Outstanding	$ 25,000,000	$ 25,000,000