CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Current Assets:
Cash	$ 73,449	$ 6,559
Receivables, Net	773,415	488,002
Unbilled Revenues	182,643	175,207
Fuel, Materials and Supplies	267,281	248,958
Regulatory Assets Current	647,615	255,144
Marketable Securities - Current	81,618	70,970
Prepayments and Other Current Assets	112,965	112,632
Total Current Assets	2,138,986	1,357,472
Property, Plant and Equipment, Net	16,303,805	10,403,065
Deferred Debits and Other Assets:
Regulatory Assets Long Term	5,008,034	3,267,710
Goodwill	3,518,454	287,591
Marketable Securities	394,207	60,311
Derivative Assets Noncurrent	93,256	98,357
Other Long-Term Assets	300,820	172,560
Total Deferred Debits and Other Assets	9,314,771	3,886,529
Total Assets	27,757,562	15,647,066
Current Liabilities:
Notes Payable to Banks	1,359,250	317,000
Long-Term Debt - Current Portion	879,746	331,582
Accounts Payable - Current	542,691	633,282
Regulatory Liability Current	226,606	167,844
Derivative Liabilities - Current	113,907	107,558
Other Liabilities - Current	678,523	390,416
Total Current Liabilities	3,800,723	1,947,682
Rate Reduction Bonds	101,347	112,260
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	3,429,538	1,868,316
Regulatory Liabilities Long-Term	561,555	266,145
Long-Term Derivative Liabilities	912,352	959,876
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	2,015,649	1,326,037
Other Long-Term Liabilities	871,428	420,011
Total Deferred Credits and Other Liabilities	7,790,522	4,840,385
Capitalization:
Long-Term Debt	6,732,536	4,614,913
Noncontrolling Interest in Consolidated Subsidiary:
Preferred Stock Not Subject to Mandatory Redemption	155,568	116,200
Common Shareholders' Equity:
Common Stock	1,662,358	980,264
Capital Surplus, Paid In	6,183,715	1,797,884
Retained Earnings	1,735,690	1,651,875
Accumulated Other Comprehensive Loss	(64,209)	(70,686)
Treasury Stock	(340,688)	(346,667)
Noncontrolling Interest	0	2,956
Common Shareholders' Equity	9,176,866	4,012,670
Total Equity	9,176,866	4,015,626
Total Capitalization	16,064,970	8,746,739
Total Liabilities and Capitalization	27,757,562	15,647,066
The Connecticut Light And Power Company [Member]
Current Assets:
Cash	6,570	1
Receivables, Net	322,159	295,028
Accounts Receivable from Affiliated Companies	1,678	1,548
Unbilled Revenues	75,246	94,995
Regulatory Assets Current	186,594	170,197
Materials and Supplies	61,704	61,102
Prepayments and Other Current Assets	53,239	53,920
Total Current Assets	707,190	676,791
Property, Plant and Equipment, Net	6,069,033	5,827,384
Deferred Debits and Other Assets:
Regulatory Assets Long Term	1,998,426	2,103,830
Derivative Assets Noncurrent	91,989	93,755
Other Long-Term Assets	92,712	89,636
Total Deferred Debits and Other Assets	2,183,127	2,287,221
Total Assets	8,959,350	8,791,396
Current Liabilities:
Notes Payable to Banks	0	31,000
Notes Payable To Affiliated Companies	372,800	58,525
Long-Term Debt - Current Portion	241,400	62,000
Accounts Payable - Current	195,315	340,321
Accounts Payable to Affiliated Companies	41,804	53,439
Obligations to Third Party Suppliers	69,222	67,967
Accrued Taxes	72,670	59,046
Regulatory Liability Current	90,208	108,291
Derivative Liabilities - Current	97,536	95,881
Other Liabilities - Current	84,529	102,065
Total Current Liabilities	1,265,484	978,535
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	1,329,397	1,215,989
Regulatory Liabilities Long-Term	130,116	139,307
Long-Term Derivative Liabilities	891,160	935,849
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	245,860	260,571
Other Long-Term Liabilities	198,083	215,640
Total Deferred Credits and Other Liabilities	2,794,616	2,767,356
Capitalization:
Long-Term Debt	2,342,806	2,521,753
Noncontrolling Interest in Consolidated Subsidiary:
Preferred Stock Not Subject to Mandatory Redemption	116,200	116,200
Common Shareholders' Equity:
Common Stock	60,352	60,352
Capital Surplus, Paid In	1,614,699	1,613,503
Retained Earnings	767,097	735,948
Accumulated Other Comprehensive Loss	(1,904)	(2,251)
Common Shareholders' Equity	2,440,244	2,407,552
Total Capitalization	4,899,250	5,045,505
Total Liabilities and Capitalization	8,959,350	8,791,396
Public Service Company Of New Hampshire [Member]
Current Assets:
Cash	3,095	56
Receivables, Net	97,815	87,545
Accounts Receivable from Affiliated Companies	1,025	1,294
Notes Receivable from Affiliated Companies	0	55,900
Unbilled Revenues	34,701	45,403
Fuel, Materials and Supplies	100,135	124,744
Regulatory Assets Current	34,767	34,178
Prepayments and Other Current Assets	21,018	35,261
Total Current Assets	292,556	384,381
Property, Plant and Equipment, Net	2,331,370	2,256,688
Deferred Debits and Other Assets:
Regulatory Assets Long Term	322,896	393,941
Other Long-Term Assets	74,261	81,531
Total Deferred Debits and Other Assets	397,157	475,472
Total Assets	3,021,083	3,116,541
Current Liabilities:
Notes Payable To Affiliated Companies	44,200
Accounts Payable - Current	61,498	106,377
Accounts Payable to Affiliated Companies	15,129	18,895
Accrued Interest	14,205	9,670
Regulatory Liability Current	18,373	24,500
Renewableportfoliostandardscomplianceobligation	13,273	12,089
Other Liabilities - Current	35,040	24,408
Total Current Liabilities	201,718	195,939
Rate Reduction Bonds	44,103	85,368
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	432,395	392,712
Regulatory Liabilities Long-Term	53,418	54,415
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	168,766	258,718
Other Long-Term Liabilities	47,936	53,304
Total Deferred Credits and Other Liabilities	702,515	759,149
Capitalization:
Long-Term Debt	997,879	997,722
Common Shareholders' Equity:
Common Stock	0
Capital Surplus, Paid In	700,841	700,285
Retained Earnings	383,961	388,910
Accumulated Other Comprehensive Loss	(9,934)	(10,832)
Common Shareholders' Equity	1,074,868	1,078,363
Total Capitalization	2,072,747	2,076,085
Total Liabilities and Capitalization	3,021,083	3,116,541
Western Massachusetts Electric Company [Member]
Current Assets:
Cash	536	1
Receivables, Net	50,950	42,757
Accounts Receivable from Affiliated Companies	294	633
Notes Receivable from Affiliated Companies	0	11,000
Unbilled Revenues	13,261	16,277
Regulatory Assets Current	34,839	35,520
Marketable Securities - Current	18,375	26,335
Prepayments and Other Current Assets	6,700	8,719
Total Current Assets	124,955	141,242
Property, Plant and Equipment, Net	1,257,318	1,077,833
Deferred Debits and Other Assets:
Regulatory Assets Long Term	213,928	233,247
Marketable Securities	39,259	30,794
Other Long-Term Assets	21,911	19,777
Total Deferred Debits and Other Assets	275,098	283,818
Total Assets	1,657,371	1,502,893
Current Liabilities:
Notes Payable To Affiliated Companies	172,500	0
Long-Term Debt - Current Portion	55,000	0
Accounts Payable - Current	61,508	111,566
Accounts Payable to Affiliated Companies	7,042	10,626
Regulatory Liability Current	22,479	33,056
Other Liabilities - Current	23,149	20,755
Total Current Liabilities	341,678	176,003
Rate Reduction Bonds	13,751	26,892
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	285,881	244,511
Regulatory Liabilities Long-Term	9,708	16,597
Pension and Other Postretirement Defined Benefit Plans, Liabilities, Noncurrent	25,516	29,546
Other Long-Term Liabilities	43,382	47,498
Total Deferred Credits and Other Liabilities	364,487	338,152
Capitalization:
Long-Term Debt	444,706	499,545
Common Shareholders' Equity:
Common Stock	10,866	10,866
Capital Surplus, Paid In	340,329	340,115
Retained Earnings	145,483	115,506
Accumulated Other Comprehensive Loss	(3,929)	(4,186)
Common Shareholders' Equity	492,749	462,301
Total Capitalization	937,455	961,846
Total Liabilities and Capitalization	1,657,371	1,502,893
NSTAR Electric Company [Member]
Current Assets:
Cash	11,734	9,373
Receivables, Net	248,648	232,828
Accounts Receivable from Affiliated Companies	146,884	389,652
Unbilled Revenues	50,465	40,380
Regulatory Assets Current	337,575	323,871
Prepayments and Other Current Assets	25,278	37,134
Total Current Assets	820,584	1,033,238
Property, Plant and Equipment, Net	4,628,823	4,447,258
Deferred Debits and Other Assets:
Regulatory Assets Long Term	1,510,087	1,680,595
Other Long-Term Assets	58,036	78,273
Total Deferred Debits and Other Assets	1,568,123	1,758,868
Total Assets	7,017,530	7,239,364
Current Liabilities:
Notes Payable to Banks	261,000	141,500
Long-Term Debt - Current Portion	1,650	400,688
Accounts Payable - Current	116,885	150,581
Accounts Payable to Affiliated Companies	229,849	514,377
Deferred Tax Liabilities Current	84,411	101,819
Regulatory Liability Current	84,476	41,579
Other Liabilities - Current	152,948	103,634
Total Current Liabilities	931,219	1,454,178
Rate Reduction Bonds	43,493	127,860
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	1,317,168	1,310,180
Regulatory Liabilities Long-Term	256,542	239,858
Other Long-Term Liabilities	180,395	195,606
Defined Benefit Pension Plan, Liabilities, Noncurrent	375,422	357,685
Due to Related Parties, Noncurrent	70,569	75,905
Total Deferred Credits and Other Liabilities	2,200,096	2,179,234
Capitalization:
Long-Term Debt	1,601,959	1,203,344
Noncontrolling Interest in Consolidated Subsidiary:
Preferred Stock Not Subject to Mandatory Redemption	43,000	43,000
Common Shareholders' Equity:
Common Stock	0	0
Capital Surplus, Paid In	992,625	992,625
Retained Earnings	1,205,138	1,239,123
Common Shareholders' Equity	2,197,763	2,231,748
Total Capitalization	3,842,722	3,478,092
Total Liabilities and Capitalization	$ 7,017,530	$ 7,239,364