CONSOLIDATED BALANCE SHEETS (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Current Assets:
Cash	$ 45,748	$ 6,559
Receivables, Net	792,822	488,002
Unbilled Revenues	216,040	175,207
Fuel, Materials and Supplies	267,713	248,958
Regulatory Assets Current	705,025	255,144
Marketable Securities - Current	91,975	70,970
Prepayments and Other Current Assets	107,972	112,632
Total Current Assets	2,227,295	1,357,472
Property, Plant and Equipment, Net	16,605,010	10,403,065
Deferred Debits and Other Assets:
Regulatory Assets Long Term	5,132,411	3,267,710
Goodwill	3,519,401	287,591
Marketable Securities	400,329	60,311
Derivative Assets Noncurrent	90,612	98,357
Other Long-Term Assets	327,766	172,560
Total Deferred Debits and Other Assets	9,470,519	3,886,529
Total Assets	28,302,824	15,647,066
Current Liabilities:
Notes Payable to Banks	1,120,196	317,000
Long-Term Debt - Current Portion	763,338	331,582
Accounts Payable - Current	764,350	633,282
Regulatory Liability Current	134,115	167,844
Derivative Liabilities - Current	117,194	107,558
Other Liabilities - Current	744,497	390,416
Total Current Liabilities	3,643,690	1,947,682
Rate Reduction Bonds	82,139	112,260
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	3,463,347	1,868,316
Regulatory Liabilities Long-Term	540,162	266,145
Long-Term Derivative Liabilities	882,654	959,876
Accrued Pension, PBOP and SERP	2,130,497	1,326,037
Other Long-Term Liabilities	967,561	420,011
Total Deferred Credits and Other Liabilities	7,984,221	4,840,385
Capitalization:
Long-Term Debt	7,200,156	4,614,913
Noncontrolling Interest in Consolidated Subsidiary:
Temporary Equity, Carrying Amount, Attributable to Noncontrolling Interest	155,568	116,200
Common Shareholders' Equity:
Common Stock	1,662,547	980,264
Capital Surplus, Paid In	6,183,267	1,797,884
Retained Earnings	1,802,714	1,651,875
Accumulated Other Comprehensive Loss	(72,854)	(70,686)
Treasury Stock	(338,624)	(346,667)
Noncontrolling Interest	0	2,956
Common Shareholders' Equity	9,237,050	4,012,670
Total Equity	9,237,050	4,015,626
Total Capitalization	16,592,774	8,746,739
Total Liabilities and Capitalization	28,302,824	15,647,066
The Connecticut Light And Power Company [Member]
Current Assets:
Cash	1	1
Receivables, Net	284,787	295,028
Accounts Receivable from Affiliated Companies	6,641	1,548
Unbilled Revenues	85,353	94,995
Regulatory Assets Current	185,858	170,197
Materials and Supplies	64,603	61,102
Prepayments and Other Current Assets	26,413	53,920
Total Current Assets	653,656	676,791
Property, Plant and Equipment, Net	6,152,959	5,827,384
Deferred Debits and Other Assets:
Regulatory Assets Long Term	2,158,363	2,103,830
Derivative Assets Noncurrent	90,612	93,755
Other Long-Term Assets	86,498	89,636
Total Deferred Debits and Other Assets	2,335,473	2,287,221
Total Assets	9,142,088	8,791,396
Current Liabilities:
Notes Payable to Banks	0	31,000
Notes Payable To Affiliated Companies	99,296	58,525
Long-Term Debt - Current Portion	125,000	62,000
Accounts Payable - Current	262,857	340,321
Accounts Payable to Affiliated Companies	52,326	53,439
Obligations to Third Party Suppliers	67,344	67,967
Accrued Taxes	60,109	59,046
Regulatory Liability Current	32,119	108,291
Derivative Liabilities - Current	96,931	95,881
Other Liabilities - Current	125,662	102,065
Total Current Liabilities	921,644	978,535
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	1,336,105	1,215,989
Regulatory Liabilities Long-Term	124,319	139,307
Long-Term Derivative Liabilities	865,571	935,849
Accrued Pension, PBOP and SERP	304,696	260,571
Other Long-Term Liabilities	197,434	215,640
Total Deferred Credits and Other Liabilities	2,828,125	2,767,356
Capitalization:
Long-Term Debt	2,737,790	2,521,753
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	1,640,149	1,613,503
Retained Earnings	839,628	735,948
Accumulated Other Comprehensive Loss	(1,800)	(2,251)
Common Shareholders' Equity	2,538,329	2,407,552
Total Capitalization	5,392,319	5,045,505
Total Liabilities and Capitalization	9,142,088	8,791,396
Public Service Company Of New Hampshire [Member]
Current Assets:
Cash	2,493	56
Receivables, Net	87,164	87,545
Accounts Receivable from Affiliated Companies	723	1,294
Notes Receivable from Affiliated Companies	0	55,900
Unbilled Revenues	39,982	45,403
Taxes Receivable	17,177	7,424
Fuel, Materials and Supplies	95,345	124,744
Regulatory Assets Current	62,882	34,178
Prepayments and Other Current Assets	22,205	27,837
Total Current Assets	327,971	384,381
Property, Plant and Equipment, Net	2,352,515	2,256,688
Deferred Debits and Other Assets:
Regulatory Assets Long Term	351,059	393,941
Other Long-Term Assets	83,052	81,531
Total Deferred Debits and Other Assets	434,111	475,472
Total Assets	3,114,597	3,116,541
Current Liabilities:
Notes Payable To Affiliated Companies	63,300	0
Accounts Payable - Current	62,864	106,377
Accounts Payable to Affiliated Companies	21,337	18,895
Accrued Interest	9,317	9,670
Regulatory Liability Current	23,002	24,500
Other Liabilities - Current	41,633	24,408
Renewable Portfolio Standards Compliance Obligation	17,383	12,089
Total Current Liabilities	238,836	195,939
Rate Reduction Bonds	29,294	85,368
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	441,577	392,712
Regulatory Liabilities Long-Term	52,418	54,415
Accrued Pension, PBOP and SERP	220,129	258,718
Other Long-Term Liabilities	47,896	53,304
Total Deferred Credits and Other Liabilities	762,020	759,149
Capitalization:
Long-Term Debt	997,932	997,722
Common Shareholders' Equity:
Common Stock	0	0
Capital Surplus, Paid In	701,052	700,285
Retained Earnings	395,118	388,910
Accumulated Other Comprehensive Loss	(9,655)	(10,832)
Common Shareholders' Equity	1,086,515	1,078,363
Total Capitalization	2,084,447	2,076,085
Total Liabilities and Capitalization	3,114,597	3,116,541
Western Massachusetts Electric Company [Member]
Current Assets:
Cash	1	1
Receivables, Net	47,297	42,757
Accounts Receivable from Affiliated Companies	164	633
Notes Receivable from Affiliated Companies	0	11,000
Unbilled Revenues	16,192	16,277
Taxes Receivable	15,513	2,263
Regulatory Assets Current	42,370	35,520
Marketable Securities - Current	27,352	26,335
Prepayments and Other Current Assets	7,963	6,456
Total Current Assets	156,852	141,242
Property, Plant and Equipment, Net	1,290,498	1,077,833
Deferred Debits and Other Assets:
Regulatory Assets Long Term	221,752	233,247
Marketable Securities	30,342	30,794
Other Long-Term Assets	23,625	19,777
Total Deferred Debits and Other Assets	275,719	283,818
Total Assets	1,723,069	1,502,893
Current Liabilities:
Notes Payable To Affiliated Companies	31,900	0
Long-Term Debt - Current Portion	55,000	0
Accounts Payable - Current	68,141	111,566
Accounts Payable to Affiliated Companies	7,103	10,626
Accrued Interest	8,304	7,714
Regulatory Liability Current	21,037	33,056
Other Liabilities - Current	24,909	13,041
Total Current Liabilities	216,394	176,003
Rate Reduction Bonds	9,352	26,892
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	303,111	244,511
Regulatory Liabilities Long-Term	9,686	16,597
Accrued Pension, PBOP and SERP	36,099	29,546
Other Long-Term Liabilities	40,148	47,498
Total Deferred Credits and Other Liabilities	389,044	338,152
Capitalization:
Long-Term Debt	550,270	499,545
Common Shareholders' Equity:
Common Stock	10,866	10,866
Capital Surplus, Paid In	390,412	340,115
Retained Earnings	160,577	115,506
Accumulated Other Comprehensive Loss	(3,846)	(4,186)
Common Shareholders' Equity	558,009	462,301
Total Capitalization	1,108,279	961,846
Total Liabilities and Capitalization	1,723,069	1,502,893
NSTAR Electric Company [Member]
Current Assets:
Cash	13,695	9,373
Receivables, Net	202,025	232,828
Accounts Receivable from Affiliated Companies	160,176	389,652
Unbilled Revenues	41,377	40,380
Regulatory Assets Current	347,081	323,871
Prepayments and Other Current Assets	28,086	34,479
Total Current Assets	792,440	1,030,583
Property, Plant and Equipment, Net	4,735,297	4,447,258
Deferred Debits and Other Assets:
Regulatory Assets Long Term	1,444,870	1,680,595
Other Long-Term Assets	87,382	81,890
Total Deferred Debits and Other Assets	1,532,252	1,762,485
Total Assets	7,059,989	7,240,326
Current Liabilities:
Notes Payable to Banks	276,000	141,500
Long-Term Debt - Current Portion	1,650	401,650
Accounts Payable - Current	168,611	150,581
Accounts Payable to Affiliated Companies	247,061	514,377
Deferred Tax Liabilities Current	104,668	101,819
Regulatory Liability Current	47,539	41,579
Other Liabilities - Current	144,433	103,634
Total Current Liabilities	989,962	1,455,140
Rate Reduction Bonds	43,493	127,860
Deferred Credits and Other Liabilities:
Accumulated Deferred Income Taxes	1,321,026	1,310,180
Regulatory Liabilities Long-Term	244,224	239,858
Accrued Pension, PBOP and SERP	360,932	357,685
Other Long-Term Liabilities	183,190	195,606
DueToRelatedPartiesNoncurrent	70,221	75,905
Total Deferred Credits and Other Liabilities	2,179,593	2,179,234
Capitalization:
Long-Term Debt	1,600,911	1,203,344
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	1,210,405	1,239,123
Common Shareholders' Equity	2,203,030	2,231,748
Total Capitalization	3,846,941	3,478,092
Total Liabilities and Capitalization	$ 7,059,989	$ 7,240,326