INCOME TAXES (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes	$ 148,193	$ 137,417
Income Tax Expense	153,226	141,545
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	408,359	379,381
Income Tax Expense	153,226	141,545
The Connecticut Light And Power Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes	19,340	59,334
Income Tax Expense	38,147	45,541
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	107,381	124,840
Income Tax Expense	38,147	45,541
NSTAR Electric Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes	41,297	1,585
Income Tax Expense	56,130	39,234
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	139,691	97,321
Income Tax Expense	56,130	39,234
Public Service Company Of New Hampshire [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes	38,767	33,667
Income Tax Expense	20,276	19,700
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	52,321	52,338
Income Tax Expense	20,276	19,700
Western Massachusetts Electric Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes	12,759	14,688
Income Tax Expense	9,113	11,558
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	22,357	29,660
Income Tax Expense	$ 9,113	$ 11,558