CONSOLIDATED STATEMENT OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Activities:
Net Income	$ 533,077	$ 400,513	$ 394,107
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Bad Debt Expense	36,275	16,420	31,352
Depreciation	519,010	302,192	300,737
Deferred Income Tax Expense (Benefit)	292,000	196,761	210,939
Pension and PBOP Expense, Net of PBOP Contributions	218,540	133,000	103,861
Pension and Other Postretirement Benefit Contributions	(295,028)	(191,101)	(90,633)
Regulatory (Underrecoveries)/Overrecoveries, Net	(259,853)	(70,863)	26,289
Amortization of Regulatory Assets/(Liabilities), Net	79,762	91,080	90,054
Amortization of Rate Reduction Bonds	142,019	69,912	232,871
Changes in Derivative Assets and Liabilities	(10,455)	(35,441)	(11,812)
Other Noncash Income (Expense)	17,032	(29,751)	(72,151)
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	(20,214)	17,570	(51,285)
Inventory	34,321	(11,033)	38,126
Taxes Receivable/(Accrued)	(5,450)	49,642	(82,103)
Change in Accounts Payable	(128,339)	18,916	(44,355)
Other Current Assets and Liabilities	8,532	12,569	17,466
Net Cash Flows Provided by Operating Activities	1,161,229	970,386	1,093,463
Investing Activities:
Investments in Property, Plant and Equipment	(1,472,272)	(1,076,730)	(954,472)
Proceeds from Sales of Marketable Securities	317,294	149,441	174,865
Purchases of Marketable Securities	(348,629)	(151,972)	(177,204)
Proceeds From Sale Of Property Plant And Equipment	0	46,841	0
Other Investing Activities	35,683	13,833	(1,157)
Net Cash Flows Used in Investing Activities	(1,467,924)	(1,018,587)	(957,968)
Financing Activities:
Cash Dividends on Common Stock	(375,047)	(194,555)	(180,542)
Cash Dividends on Preferred Stock	(7,029)	(5,559)	(5,559)
Increase/(Decrease) in Short-Term Debt	825,000	50,000	166,687
Issuance of Long-Term Debt	850,000	627,500	145,000
Retirements of Long-Term Debt	(839,136)	(369,586)	(4,286)
Retirements of Rate Reduction Bonds	(114,433)	(69,312)	(260,864)
Other Financing Activities	6,529	(7,123)	512
Net Cash Flows Provided by/(Used in) Financing Activities	345,884	31,365	(139,052)
Net Increase/(Decrease) in Cash	39,189	(16,836)	(3,557)
Cash - Beginning of Year	6,559	23,395	26,952
Cash - End of Year	45,748	6,559	23,395
The Connecticut Light And Power Company [Member]
Operating Activities:
Net Income	209,725	250,164	244,143
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Bad Debt Expense	2,080	3,215	7,484
Depreciation	166,853	157,747	172,167
Deferred Income Tax Expense (Benefit)	140,993	112,620	115,069
Pension and PBOP Expense, Net of PBOP Contributions	24,062	10,664	1,595
Regulatory (Underrecoveries)/Overrecoveries, Net	(100,505)	(82,502)	37,528
Amortization of Regulatory Assets/(Liabilities), Net	14,372	61,025	78,870
Amortization of Rate Reduction Bonds	0	0	167,021
Other Noncash Income (Expense)	(31,032)	(36,928)	(55,515)
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	(7,741)	14,610	1,895
Inventory	(4,573)	(2,206)	3,377
Taxes Receivable/(Accrued)	15,702	2,719	(56,002)
Change in Accounts Payable	(190,240)	8,864	(35,976)
Other Current Assets and Liabilities	(27,803)	13,291	15,649
Net Cash Flows Provided by Operating Activities	211,893	513,283	697,305
Investing Activities:
Investments in Property, Plant and Equipment	(449,137)	(424,865)	(380,304)
(Increase) Decrease In Notes Receivable From Affiliated Company	0	0	97,775
Proceeds From Sale Of Property Plant And Equipment	0	46,841	0
Other Investing Activities	32,009	16,001	5,385
Net Cash Flows Used in Investing Activities	(417,128)	(362,023)	(277,144)
Financing Activities:
Cash Dividends on Common Stock	(100,486)	(243,218)	(217,691)
Cash Dividends on Preferred Stock	(5,559)	(5,559)	(5,559)
Increase/(Decrease) in Short-Term Debt	58,000	31,000	0
Issuance of Long-Term Debt	0	245,500	0
Increase/(Decrease) in Notes Payable to Affiliate	346,575	52,300	6,225
Retirements of Long-Term Debt	(116,400)	(245,500)	0
Retirements of Rate Reduction Bonds	0	0	(195,587)
Capital Contributions from NU Parent	25,000	6,748	2,513
Other Financing Activities	(1,895)	(2,292)	(345)
Net Cash Flows Provided by/(Used in) Financing Activities	205,235	(161,021)	(410,444)
Net Increase/(Decrease) in Cash	0	(9,761)	9,717
Cash - Beginning of Year	1	9,762	45
Cash - End of Year	1	1	9,762
Public Service Company Of New Hampshire [Member]
Operating Activities:
Net Income	96,882	100,267	90,067
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Bad Debt Expense	6,457	7,035	8,858
Depreciation	87,602	76,167	67,237
Deferred Income Tax Expense (Benefit)	58,552	75,628	39,225
Pension and PBOP Expense, Net of PBOP Contributions	26,312	27,298	29,112
Pension and Other Postretirement Benefit Contributions	(96,880)	(121,178)	(53,689)
Regulatory (Underrecoveries)/Overrecoveries, Net	(183)	6,079	(2,834)
Amortization of Regulatory Assets/(Liabilities), Net	(24,086)	25,383	11,232
Amortization of Rate Reduction Bonds	56,645	53,389	50,357
Changes in Derivative Assets and Liabilities	0	(18,072)	0
Other Noncash Income (Expense)	4,748	(20,958)	(31,590)
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	(84)	7,833	(24,497)
Inventory	25,897	(9,873)	14,891
Taxes Receivable/(Accrued)	(9,752)	5,139	10,037
Change in Accounts Payable	(15,248)	(4,517)	(14,427)
Other Current Assets and Liabilities	13,436	(4,915)	1,294
Net Cash Flows Provided by Operating Activities	230,298	204,705	195,273
Investing Activities:
Investments in Property, Plant and Equipment	(203,902)	(241,772)	(296,335)
(Increase) Decrease In Notes Receivable From Affiliated Company	55,900	(55,900)	0
Other Investing Activities	4,065	2,089	(7,819)
Net Cash Flows Used in Investing Activities	(143,937)	(295,583)	(304,154)
Financing Activities:
Cash Dividends on Common Stock	(90,674)	(58,828)	(50,584)
Increase/(Decrease) in Short-Term Debt	0	(30,000)	30,000
Issuance of Long-Term Debt	0	282,000	0
Increase/(Decrease) in Notes Payable to Affiliate	63,300	(47,900)	21,200
Retirements of Long-Term Debt	0	(119,800)	0
Retirements of Rate Reduction Bonds	(56,074)	(52,879)	(49,867)
Capital Contributions from NU Parent	0	120,030	158,969
Other Financing Activities	(476)	(4,248)	(252)
Net Cash Flows Provided by/(Used in) Financing Activities	(83,924)	88,375	109,466
Net Increase/(Decrease) in Cash	2,437	(2,503)	585
Cash - Beginning of Year	56	2,559	1,974
Cash - End of Year	2,493	56	2,559
Western Massachusetts Electric Company [Member]
Operating Activities:
Net Income	54,503	43,054	23,090
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Bad Debt Expense	2,294	3,133	9,747
Depreciation	29,971	26,455	23,561
Deferred Income Tax Expense (Benefit)	53,942	23,056	10,963
Regulatory (Underrecoveries)/Overrecoveries, Net	(19,152)	3,328	(11,048)
Amortization of Regulatory Assets/(Liabilities), Net	410	4,492	1,892
Amortization of Rate Reduction Bonds	17,632	16,523	15,494
Changes in Derivative Assets and Liabilities	0	(6,859)	0
Other Noncash Income (Expense)	(6,248)	(3,719)	(7,567)
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	(8,896)	(7,263)	(6,838)
Inventory	(2,882)	331	4,650
Taxes Receivable/(Accrued)	(8,311)	5,084	(393)
Change in Accounts Payable	(19,297)	12,956	(92)
Other Current Assets and Liabilities	581	3,824	2,406
Net Cash Flows Provided by Operating Activities	94,547	124,395	65,865
Investing Activities:
Investments in Property, Plant and Equipment	(264,175)	(237,996)	(115,178)
Proceeds from Sales of Marketable Securities	79,769	125,157	114,191
Purchases of Marketable Securities	(80,529)	(125,453)	(114,587)
(Increase) Decrease In Notes Receivable From Affiliated Company	11,000	(11,000)	0
Other Investing Activities	(28)	(1,919)	(888)
Net Cash Flows Used in Investing Activities	(253,963)	(251,211)	(116,462)
Financing Activities:
Cash Dividends on Common Stock	(9,432)	(26,305)	(14,882)
Issuance of Long-Term Debt	150,000	100,000	95,000
Increase/(Decrease) in Notes Payable to Affiliate	31,900	(20,400)	(115,700)
Retirements of Long-Term Debt	(53,800)	0	0
Retirements of Rate Reduction Bonds	(17,540)	(16,433)	(15,410)
Capital Contributions from NU Parent	50,000	91,812	102,479
Other Financing Activities	8,288	(1,858)	(890)
Net Cash Flows Provided by/(Used in) Financing Activities	159,416	126,816	50,597
Net Increase/(Decrease) in Cash	0	0	0
Cash - Beginning of Year	1	1	1
Cash - End of Year	1	1	1
NSTAR Electric Company [Member]
Operating Activities:
Net Income	190,242	252,494	248,575
Adjustments to Reconcile Net Income to Net Cash Flows Provided by Operating Activities:
Bad Debt Expense	40,301	22,582	29,417
Depreciation	171,070	163,368	158,574
Deferred Income Tax Expense (Benefit)	4,264	72,006	41,612
Pension and PBOP Expense, Net of PBOP Contributions	66,010	54,704	60,528
Pension Contribution	(25,000)	(125,000)	(25,000)
Regulatory (Underrecoveries)/Overrecoveries, Net	(16,129)	68,353	95,532
Amortization of Regulatory Assets/(Liabilities), Net	117,682	82,979	19,071
Amortization of Rate Reduction Bonds	90,322	90,322	104,481
Other Noncash Income (Expense)	(32,048)	539	(76,866)
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	(10,496)	(26,041)	(37,132)
Inventory	1,813	(12,968)	3,077
Taxes Receivable/(Accrued)	29,899	149,889	(20,270)
Change in Accounts Payable	(59,217)	(53,939)	37,661
Other Current Assets and Liabilities	22,568	7,040	(93,528)
Net Cash Flows Provided by Operating Activities	591,281	746,328	545,732
Investing Activities:
Investments in Property, Plant and Equipment	(414,089)	(390,427)	(317,046)
Other Investing Activities	3,460	3,363	26,382
Net Cash Flows Used in Investing Activities	(410,629)	(387,064)	(290,664)
Financing Activities:
Cash Dividends on Common Stock	(217,000)	(169,900)	(188,200)
Cash Dividends on Preferred Stock	(1,960)	(1,960)	(1,960)
Increase/(Decrease) in Short-Term Debt	134,500	(86,000)	(113,500)
Issuance of Long-Term Debt	400,000	0	300,000
Retirements of Long-Term Debt	(401,650)	(16,650)	(126,648)
Retirements of Rate Reduction Bonds	(84,367)	(84,346)	(119,014)
Other Financing Activities	(5,853)	0	(7,854)
Net Cash Flows Provided by/(Used in) Financing Activities	(176,330)	(358,856)	(257,176)
Net Increase/(Decrease) in Cash	4,322	408	(2,108)
Cash - Beginning of Year	9,373	8,965	11,073
Cash - End of Year	$ 13,695	$ 9,373	$ 8,965