COMMON SHAREHOLDER EQUIT AND NCI (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Shareholders' Equity, Beginning Balance				$ 9,611,528,000					$ 9,611,528,000
Net Income	223,600,000	236,500,000	129,200,000	237,800,000	179,200,000	211,400,000	173,100,000	230,000,000	827,065,000	793,689,000	533,077,000
Net Income Attributable to Noncontrolling Interests									7,519,000	7,682,000	7,132,000
Other Comprehensive Income (Loss), Net of Tax									(27,978,000)	26,823,000	(2,168,000)
Shareholders' Equity, Ending Balance	9,976,815,000				9,611,528,000				9,976,815,000	9,611,528,000
Common Shareholders Equity [Member]
Shareholders' Equity, Beginning Balance											4,012,700,000
Net Income									827,065,000	793,689,000	3,359,000
Dividends on Common Shares									(496,524,000)	11,078,000	(375,527,000)
Dividends on Preferred Stock									(7,519,000)	(10,748,000)	(7,029,000)
Issuance of Common Shares									6,609,000	29,468,000	13,327,000
Other Transcations, Net									63,600,000	13,200,000	21,100,000
Net Income Attributable to Noncontrolling Interests											(103,000)
Other Comprehensive Income (Loss), Net of Tax									(27,978,000)		(2,168,000)
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		0	3,000,000
Contributions To Northern Pass Trans											300,000
Net Income Attributable to Noncontrolling Interests										0	100,000
Shareholders' Equity, Ending Balance					0					0	0
Other Impacts of Merger with NSTAR											(3,400,000)
Total Equity [Member]
Shareholders' Equity, Beginning Balance				9,611,500,000				9,237,100,000	9,611,500,000	9,237,100,000	4,015,700,000
Net Income									827,100,000	793,700,000	533,100,000
Dividends on Common Shares									(496,500,000)	(462,700,000)	(375,500,000)
Dividends on Preferred Stock									(7,500,000)	(7,700,000)	(7,000,000)
Issuance of Common Shares									6,600,000	11,100,000	13,300,000
Contributions To Northern Pass Trans											300,000
Other Transcations, Net									63,600,000	13,200,000	21,100,000
Net Income Attributable to Noncontrolling Interests									0
Other Comprehensive Income (Loss), Net of Tax									(28,000,000)	26,800,000	(2,200,000)
Shareholders' Equity, Ending Balance	9,976,800,000				9,611,500,000				9,976,800,000	9,611,500,000	9,237,100,000
Purchase Price of NSTAR											5,038,300,000
Preferred Stock Member
Shareholders' Equity, Beginning Balance				155,600,000				155,600,000	155,600,000	155,600,000	116,200,000
Dividends on Preferred Stock									(7,500,000)	(7,700,000)	(7,000,000)
Net Income Attributable to Noncontrolling Interests									7,500,000	7,700,000	7,000,000
Shareholders' Equity, Ending Balance	155,600,000				155,600,000				155,600,000	155,600,000	155,600,000
Other Impacts of Merger with NSTAR											$ 39,400,000