COMMON SHAREHOLDER EQUIT AND NCI (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Shareholders' Equity, Beginning Balance				$ 9,237,050,000					$ 9,237,050,000
Net Income	179,200,000	211,400,000	173,100,000	230,000,000	176,600,000	209,500,000	46,200,000	100,800,000	793,689,000	533,077,000	400,513,000
Net Income Attributable to Noncontrolling Interests									7,682,000	7,132,000	5,820,000
Other Comprehensive Income (Loss), Net of Tax									26,823,000	(2,168,000)	(27,316,000)
Shareholders' Equity, Ending Balance	9,611,528,000				9,237,050,000				9,611,528,000	9,237,050,000
Other Impacts of Merger with NSTAR											39,400,000
Common Shareholders Equity [Member]
Shareholders' Equity, Beginning Balance				9,237,100,000				4,012,700,000	9,237,100,000	4,012,700,000	3,811,200,000
Net Income									793,689,000	533,077,000	400,513,000
Dividends on Common Shares									(462,741,000)	(375,527,000)	(195,595,000)
Dividends, Preferred Stock									(7,682,000)	(7,029,000)	(5,559,000)
Issuance of Common Shares									11,078,000	13,327,000	5,851,000
Other Transcations, Net									13,200,000	21,100,000	23,900,000
Net Income Attributable to Noncontrolling Interests										(103,000)	(261,000)
Other Comprehensive Income (Loss), Net of Tax									26,823,000	(2,168,000)	(27,316,000)
Shareholders' Equity, Ending Balance	9,611,500,000				9,237,100,000				9,611,500,000	9,237,100,000	4,012,700,000
Purchase Price of NSTAR										5,038,300,000
Other Impacts of Merger with NSTAR										3,400,000
Noncontrolling Interest Member
Shareholders' Equity, Beginning Balance				0				3,000,000	0	3,000,000
Contributions To Northern Pass Trans										300,000	1,200,000
Net Income Attributable to Noncontrolling Interests									0	100,000	300,000
Shareholders' Equity, Ending Balance	0				0				0	0	3,000,000
Other Impacts of Merger with NSTAR										(3,400,000)
Total Equity [Member]
Shareholders' Equity, Beginning Balance				9,237,100,000				4,015,700,000	9,237,100,000	4,015,700,000	3,812,700,000
Net Income									793,700,000	533,100,000	400,500,000
Dividends on Common Shares									(462,700,000)	(375,500,000)	(195,600,000)
Dividends, Preferred Stock									(7,700,000)	(7,000,000)	(5,600,000)
Issuance of Common Shares									11,100,000	13,300,000	5,900,000
Contributions To Northern Pass Trans										300,000	1,200,000
Other Transcations, Net									13,200,000	21,100,000	23,900,000
Net Income Attributable to Noncontrolling Interests									0
Other Comprehensive Income (Loss), Net of Tax									26,800,000	(2,200,000)	(27,300,000)
Shareholders' Equity, Ending Balance	9,611,500,000				9,237,100,000				9,611,500,000	9,237,100,000	4,015,700,000
Purchase Price of NSTAR										5,038,300,000
Preferred Stock Member
Shareholders' Equity, Beginning Balance				155,600,000				116,200,000	155,600,000	116,200,000	116,200,000
Dividends, Preferred Stock									(7,700,000)	(7,000,000)	(5,600,000)
Net Income Attributable to Noncontrolling Interests									7,700,000	7,000,000	5,600,000
Shareholders' Equity, Ending Balance	$ 155,600,000				$ 155,600,000				$ 155,600,000	$ 155,600,000	$ 116,200,000