CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current Assets:
Cash	$ 60,817	$ 45,748
Receivables, Net	832,989	792,822
Unbilled Revenues	195,870	216,040
Fuel, Materials and Supplies	239,226	267,713
Regulatory Assets Current	625,542	705,025
Marketable Securities - Current	99,115	91,975
Prepayments and Other Current Assets	104,452	107,972
Total Current Assets	2,158,011	2,227,295
Property, Plant and Equipment, Net	16,737,549	16,605,010
Deferred Debits and Other Assets:
Regulatory Assets Long Term	5,015,108	5,132,411
Goodwill	3,519,401	3,519,401
Marketable Securities	492,003	400,329
Derivative Assets Noncurrent	94,970	90,612
Other Long-Term Assets	311,619	327,766
Total Deferred Debits and Other Assets	9,433,101	9,470,519
Total Assets	28,328,661	28,302,824
Current Liabilities:
Notes Payable to Banks	1,287,000	1,120,196
Long-Term Debt - Current Portion	947,328	763,338
Accounts Payable - Current	633,458	764,350
Regulatory Liability Current	191,707	134,115
Derivative Liabilities - Current	108,964	117,194
Other Liabilities - Current	671,180	744,497
Total Current Liabilities	3,839,637	3,643,690
Rate Reduction Bonds	19,610	82,139
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	3,614,278	3,463,347
Regulatory Liabilities Long-Term	531,546	540,162
Long-Term Derivative Liabilities	840,043	882,654
Accrued Pension, PBOP and SERP	2,097,550	2,130,497
Other Long-Term Liabilities	873,637	967,561
Total Deferred Credits and Other Liabilities	7,957,054	7,984,221
Capitalization:
Long-Term Debt	7,011,561	7,200,156
Noncontrolling Interest in Consolidated Subsidiary:
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	155,568	155,568
Common Shareholders' Equity:
Common Stock	1,664,760	1,662,547
Capital Surplus, Paid In	6,173,801	6,183,267
Retained Earnings	1,914,371	1,802,714
Accumulated Other Comprehensive Loss	(70,898)	(72,854)
Treasury Stock	(336,803)	(338,624)
Common Shareholders' Equity	9,345,231	9,237,050
Total Capitalization	16,512,360	16,592,774
Total Liabilities and Capitalization	28,328,661	28,302,824
The Connecticut Light And Power Company [Member]
Current Assets:
Cash	8,307	1
Receivables, Net	318,111	284,787
Accounts Receivable from Affiliated Companies	2,942	6,641
Unbilled Revenues	80,590	85,353
Regulatory Assets Current	172,883	185,858
Materials and Supplies	63,035	64,603
Prepayments and Other Current Assets	47,906	26,413
Total Current Assets	693,774	653,656
Property, Plant and Equipment, Net	6,184,628	6,152,959
Deferred Debits and Other Assets:
Regulatory Assets Long Term	2,141,137	2,158,363
Derivative Assets Noncurrent	94,970	90,612
Other Long-Term Assets	87,368	86,498
Total Deferred Debits and Other Assets	2,323,475	2,335,473
Total Assets	9,201,877	9,142,088
Current Liabilities:
Notes Payable To Affiliated Companies	210,400	99,296
Long-Term Debt - Current Portion	125,000	125,000
Accounts Payable - Current	196,526	262,857
Accounts Payable to Affiliated Companies	31,968	52,326
Obligations to Third Party Suppliers	67,833	67,344
Accrued Taxes	53,383	60,109
Regulatory Liability Current	42,324	32,119
Derivative Liabilities - Current	95,573	96,931
Other Liabilities - Current	102,094	125,662
Total Current Liabilities	925,101	921,644
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	1,401,609	1,336,105
Regulatory Liabilities Long-Term	110,688	124,319
Long-Term Derivative Liabilities	826,023	865,571
Accrued Pension, PBOP and SERP	304,549	304,696
Other Long-Term Liabilities	192,624	197,434
Total Deferred Credits and Other Liabilities	2,835,493	2,828,125
Capitalization:
Long-Term Debt	2,740,614	2,737,790
Noncontrolling Interest in Consolidated Subsidiary:
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	116,200	116,200
Common Shareholders' Equity:
Common Stock	60,352	60,352
Capital Surplus, Paid In	1,640,566	1,640,149
Retained Earnings	885,246	839,628
Accumulated Other Comprehensive Loss	(1,695)	(1,800)
Common Shareholders' Equity	2,584,469	2,538,329
Total Capitalization	5,441,283	5,392,319
Total Liabilities and Capitalization	9,201,877	9,142,088
Public Service Company Of New Hampshire [Member]
Current Assets:
Cash	3,376	2,493
Receivables, Net	86,225	87,164
Accounts Receivable from Affiliated Companies	7,568	723
Unbilled Revenues	39,062	39,982
Taxes Receivable	2,020	17,177
Fuel, Materials and Supplies	94,783	95,345
Regulatory Assets Current	62,184	62,882
Prepayments and Other Current Assets	8,127	22,205
Total Current Assets	303,345	327,971
Property, Plant and Equipment, Net	2,360,753	2,352,515
Deferred Debits and Other Assets:
Regulatory Assets Long Term	329,314	351,059
Other Long-Term Assets	82,284	83,052
Total Deferred Debits and Other Assets	411,598	434,111
Total Assets	3,075,696	3,114,597
Current Liabilities:
Notes Payable To Affiliated Companies	53,400	63,300
Long-Term Debt - Current Portion	108,985
Accounts Payable - Current	64,619	62,864
Accounts Payable to Affiliated Companies	12,624	21,337
Accrued Interest	13,840	9,317
Regulatory Liability Current	18,132	23,002
Other Liabilities - Current	43,941	41,633
Renewable Portfolio Standards Compliance Obligation	21,946	17,383
Total Current Liabilities	337,487	238,836
Rate Reduction Bonds	14,974	29,294
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	449,979	441,577
Regulatory Liabilities Long-Term	52,473	52,418
Accrued Pension, PBOP and SERP	186,836	220,129
Other Long-Term Liabilities	45,972	47,896
Total Deferred Credits and Other Liabilities	735,260	762,020
Capitalization:
Long-Term Debt	888,999	997,932
Common Shareholders' Equity:
Common Stock	0	0
Capital Surplus, Paid In	701,241	701,052
Retained Earnings	407,113	395,118
Accumulated Other Comprehensive Loss	(9,378)	(9,655)
Common Shareholders' Equity	1,098,976	1,086,515
Total Capitalization	1,987,975	2,084,447
Total Liabilities and Capitalization	3,075,696	3,114,597
Western Massachusetts Electric Company [Member]
Current Assets:
Cash	1,768	1
Receivables, Net	48,113	47,297
Accounts Receivable from Affiliated Companies	154	164
Unbilled Revenues	16,071	16,192
Taxes Receivable	3	15,513
Regulatory Assets Current	43,534	42,370
Marketable Securities - Current	30,588	27,352
Prepayments and Other Current Assets	7,571	7,963
Total Current Assets	147,802	156,852
Property, Plant and Equipment, Net	1,305,743	1,290,498
Deferred Debits and Other Assets:
Regulatory Assets Long Term	207,901	221,752
Marketable Securities	27,169	30,342
Other Long-Term Assets	24,018	23,625
Total Deferred Debits and Other Assets	259,088	275,719
Total Assets	1,712,633	1,723,069
Current Liabilities:
Notes Payable To Affiliated Companies	43,400	31,900
Long-Term Debt - Current Portion	55,000	55,000
Accounts Payable - Current	44,303	68,141
Accounts Payable to Affiliated Companies	3,061	7,103
Accrued Interest	1,918	8,304
Regulatory Liability Current	15,638	21,037
Other Liabilities - Current	32,756	24,909
Total Current Liabilities	196,076	216,394
Rate Reduction Bonds	4,636	9,352
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	316,727	303,111
Regulatory Liabilities Long-Term	9,736	9,686
Accrued Pension, PBOP and SERP	35,431	36,099
Other Long-Term Liabilities	33,203	40,148
Total Deferred Credits and Other Liabilities	395,097	389,044
Capitalization:
Long-Term Debt	550,057	550,270
Common Shareholders' Equity:
Common Stock	10,866	10,866
Capital Surplus, Paid In	390,485	390,412
Retained Earnings	169,179	160,577
Accumulated Other Comprehensive Loss	(3,763)	(3,846)
Common Shareholders' Equity	566,767	558,009
Total Capitalization	1,116,824	1,108,279
Total Liabilities and Capitalization	1,712,633	1,723,069
NSTAR Electric Company [Member]
Current Assets:
Cash	17,109	13,695
Receivables, Net	233,438	202,025
Accounts Receivable from Affiliated Companies	303,699	160,176
Unbilled Revenues	35,895	41,377
Regulatory Assets Current	309,559	347,081
Prepayments and Other Current Assets	42,425	28,086
Total Current Assets	942,125	792,440
Property, Plant and Equipment, Net	4,788,158	4,735,297
Deferred Debits and Other Assets:
Regulatory Assets Long Term	1,489,317	1,444,870
Other Long-Term Assets	57,036	87,382
Total Deferred Debits and Other Assets	1,546,353	1,532,252
Total Assets	7,276,636	7,059,989
Current Liabilities:
Notes Payable to Banks	308,000	276,000
Long-Term Debt - Current Portion	1,650	1,650
Accounts Payable - Current	205,738	168,611
Accounts Payable to Affiliated Companies	353,037	247,061
Deferred Tax Liabilities Current	84,096	104,668
Regulatory Liability Current	67,277	47,539
Other Liabilities - Current	148,360	144,433
Total Current Liabilities	1,168,158	989,962
Rate Reduction Bonds	0	43,493
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	1,365,951	1,321,026
Regulatory Liabilities Long-Term	248,376	244,224
Accrued Pension, PBOP and SERP	362,432	360,932
Other Long-Term Liabilities	171,439	183,190
DueToRelatedPartiesNoncurrent	65,668	70,221
Total Deferred Credits and Other Liabilities	2,213,866	2,179,593
Capitalization:
Long-Term Debt	1,600,950	1,600,911
Noncontrolling Interest in Consolidated Subsidiary:
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	43,000	43,000
Common Shareholders' Equity:
Common Stock	0	0
Capital Surplus, Paid In	992,625	992,625
Retained Earnings	1,258,037	1,210,405
Common Shareholders' Equity	2,250,662	2,203,030
Total Capitalization	3,894,612	3,846,941
Total Liabilities and Capitalization	$ 7,276,636	$ 7,059,989