INCOME TAXES (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes			$ 64,755	$ 334,225
Income Tax Expense	126,539	109,351	345,858	325,442
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	363,032	320,731	949,434	939,855
Income Tax Expense	126,539	109,351	345,858	325,442
The Connecticut Light And Power Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes			(14,400)	89,084
Income Tax Expense	30,038	36,136	95,980	113,149
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	113,914	102,459	296,540	332,337
Income Tax Expense	30,038	36,136	95,980	113,149
Other Tax Disclosures [Abstract]
Other Information Pertaining To Income Taxes			In the third quarter of 2014, the Company recorded a reduction to its state credit carryforwards of $11 million (CL&P $10.1 million), net of tax, as a result of an update to reflect the amounts expired. Further, the Company decreased its valuation allowance reserve for state credits by $22.3 million (all at CL&P), net of tax, to reflect the expiration of state credits in its recently filed return and the latest available data. These updates resulted in a net reduction in income tax expense of $11.3 million (CL&P $12.2 million).
NSTAR Electric Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes			(39,399)	26,358
Income Tax Expense	76,975	68,558	156,655	137,499
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	192,549	175,567	390,446	350,661
Income Tax Expense	76,975	68,558	156,655	137,499
Public Service Company Of New Hampshire [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes			67,827	57,066
Income Tax Expense	17,603	18,196	52,199	52,797
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	45,839	46,549	137,154	137,340
Income Tax Expense	17,603	18,196	52,199	52,797
Western Massachusetts Electric Company [Member]
Deferred Income Tax Expense (Benefit), Continuing Operations [Abstract]
Total Deferred Income Taxes			13,479	79,401
Income Tax Expense	10,490	8,588	26,596	30,424
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Income Before Income Tax Expense	25,155	23,613	66,381	80,465
Income Tax Expense	$ 10,490	$ 8,588	$ 26,596	$ 30,424