Equity Method Investments (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	$ 1,400	$ 1,400
NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	600	600
Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	400	400
Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	300	300
CYAPC [Member] | The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	34.50%
CYAPC [Member] | NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	14.00%
CYAPC [Member] | Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	9.50%
CYAPC [Member] | Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	5.00%
YAEC [Member] | The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	24.50%
YAEC [Member] | NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	14.00%
YAEC [Member] | Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	7.00%
YAEC [Member] | Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	7.00%
MYAPC [Member] | The Connecticut Light And Power Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	12.00%
MYAPC [Member] | NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	4.00%
MYAPC [Member] | Western Massachusetts Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	3.00%
MYAPC [Member] | Public Service Company Of New Hampshire [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage	5.00%
Regional Transmission Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	6,000	4,600
Equity Method Investment, Ownership Percentage	37.20%	22.70%
Regional Transmission Company [Member] | NSTAR Electric Company [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	2,300	3,000
Equity Method Investment, Ownership Percentage	14.50%
Energy Investment Fund [Member]
Schedule of Equity Method Investments [Line Items]
Equity Method Investments	$ 6,800	$ 4,200