CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Activities:
Net Income	$ 949,821	$ 886,004	$ 827,065
Provided by Operating Activities:
Depreciation	715,466	665,856	614,657
Deferred Income Taxes	466,463	491,736	443,259
Pension, SERP and PBOP Expense	39,912	96,017	99,056
Pension and PBOP Contributions	(158,741)	(162,452)	(211,649)
Regulatory Over/(Under) Recoveries, Net	13,340	(163,287)	6,853
Amortization of Regulatory Assets, Net	71,696	22,339	10,704
Refunds/(Payments) Related to Spent Nuclear Fuel, Net	59,804	(297,253)	132,138
Other	(77,294)	(82,219)	56,026
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	(142,699)	(39,797)	(122,139)
Fuel, Materials, Supplies and Inventory	7,755	34,112	(41,310)
Taxes Receivable/Accrued, Net	234,543	30,282	(323,224)
Accounts Payable	(14,126)	(91,618)	144,743
Other Current Assets and Liabilities, Net	9,112	44,031	15,797
Net Cash Flows Provided by Operating Activities	2,175,052	1,433,751	1,651,976
Investing Activities:
Investments in Property, Plant and Equipment	(1,976,867)	(1,724,139)	(1,603,744)
Proceeds from Sales of Marketable Securities	659,338	799,165	488,789
Purchases of Marketable Securities	(681,272)	(717,114)	(491,220)
Payments to Acquire Investments	(188,958)	(23,353)	(9,779)
Other Investing Activities	36,951	6,291	24,159
Net Cash Flows Used in Investing Activities	(2,150,808)	(1,659,150)	(1,591,795)
Financing Activities:
Cash Dividends on Common Shares	(564,486)	(529,791)	(475,227)
Cash Dividends on Preferred Stock	(7,519)	(7,519)	(7,519)
Increase/(Decrease) in Short-Term Debt	(12,453)	(242,122)	285,075
Issuance of Long-Term Debt	800,000	1,225,000	725,000
Retirements of Long-Term Debt	(200,000)	(216,700)	(576,551)
Other Financing Activities	(33,482)	(18,225)	(15,620)
Net Cash Flows (Used in)/Provided by Financing Activities	(17,940)	210,643	(64,842)
Net Increase/(Decrease) in Cash and Cash Equivalents	6,304	(14,756)	(4,661)
Cash and Cash Equivalents - Beginning of Year	23,947	38,703	43,364
Cash and Cash Equivalents - End of Year	30,251	23,947	38,703
The Connecticut Light And Power Company [Member]
Operating Activities:
Net Income	334,254	299,360	287,754
Provided by Operating Activities:
Depreciation	230,489	215,289	188,837
Deferred Income Taxes	168,919	135,994	130,949
Pension, SERP and PBOP Expense	6,948	14,091	14,992
Regulatory Over/(Under) Recoveries, Net	(68,730)	(53,781)	(20,502)
Amortization of Regulatory Assets, Net	38,765	12,318	59,336
Refunds/(Payments) Related to Spent Nuclear Fuel, Net	13,568	(242,231)	68,610
Other	(32,212)	(36,385)	(1,342)
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	3,229	(29,195)	(78,631)
Fuel, Materials, Supplies and Inventory	(8,926)	22,810	13,063
Taxes Receivable/Accrued, Net	123,692	(13,517)	(126,376)
Accounts Payable	3,252	(16,910)	68,891
Other Current Assets and Liabilities, Net	(1,770)	(9,514)	6,838
Net Cash Flows Provided by Operating Activities	811,478	298,329	612,419
Investing Activities:
Investments in Property, Plant and Equipment	(611,984)	(523,849)	(515,710)
Proceeds from the Sale of Property, Plant and Equipment	9,047	0	4,918
Other Investing Activities	296	(716)	7,735
Net Cash Flows Used in Investing Activities	(602,641)	(524,565)	(503,057)
Financing Activities:
Cash Dividends on Common Shares	(199,599)	(196,000)	(171,200)
Cash Dividends on Preferred Stock	(5,559)	(5,559)	(5,559)
(Decrease)/Increase in Notes Payable to Eversource Parent	(197,300)	144,000	(153,900)
Issuance of Long-Term Debt	0	350,000	250,000
Retirements of Long-Term Debt	0	(162,000)	(150,000)
Capital Contributions from Eversource Parent	200,000	105,000	120,000
Other Financing Activities	(857)	(10,504)	(3,584)
Net Cash Flows (Used in)/Provided by Financing Activities	(203,315)	224,937	(114,243)
Net Increase/(Decrease) in Cash and Cash Equivalents	5,522	(1,299)	(4,881)
Cash and Cash Equivalents - Beginning of Year	1,057	2,356	7,237
Cash and Cash Equivalents - End of Year	6,579	1,057	2,356
NSTAR Electric Company [Member]
Operating Activities:
Net Income	292,705	344,542	303,088
Provided by Operating Activities:
Depreciation	213,115	196,770	188,693
Deferred Income Taxes	80,200	173,155	108,133
Pension, SERP and PBOP Expense	(327)	10,786	6,760
Pension and PBOP Contributions	(37,305)	(9,886)	(120,306)
Regulatory Over/(Under) Recoveries, Net	117,795	(124,323)	57,696
Amortization of Regulatory Assets/(Liabilities), Net	30,083	(12,989)	(6,330)
Bad Debt Expense	27,978	14,228	24,740
Refunds/(Payments) Related to Spent Nuclear Fuel, Net	4,983	783	30,193
Other	(52,326)	(56,063)	(51,478)
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	(58,591)	(35,525)	(18,853)
Fuel, Materials, Supplies and Inventory	13,198	406	(29,943)
Taxes Receivable/Accrued, Net	27,434	77,429	(122,746)
Accounts Payable	(32,969)	21,961	9,753
Accounts Receivable from/Payable to Affiliates, Net	48,337	11,450	115,092
Other Current Assets and Liabilities, Net	(32,920)	44,302	38,535
Net Cash Flows Provided by Operating Activities	641,390	657,026	533,027
Investing Activities:
Investments in Property, Plant and Equipment	(524,306)	(469,466)	(465,028)
Net Cash Flows Used in Investing Activities	(524,306)	(469,466)	(465,028)
Financing Activities:
Cash Dividends on Common Shares	(278,299)	(197,999)	(253,000)
Cash Dividends on Preferred Stock	(1,960)	(1,960)	(1,961)
Increase/(Decrease) in Short-Term Debt	64,000	(239,500)	198,500
Issuance of Long-Term Debt	250,000	250,000	300,000
Retirements of Long-Term Debt	(200,000)	(4,700)	(301,650)
Capital Contributions from Eversource Parent	50,000	0	0
Other Financing Activities	(677)	(2,828)	(5,136)
Net Cash Flows (Used in)/Provided by Financing Activities	(116,936)	(196,987)	(63,247)
Net Increase/(Decrease) in Cash and Cash Equivalents	148	(9,427)	4,752
Cash and Cash Equivalents - Beginning of Year	3,346	12,773	8,021
Cash and Cash Equivalents - End of Year	3,494	3,346	12,773
Public Service Company Of New Hampshire [Member]
Operating Activities:
Net Income	131,985	114,442	113,944
Provided by Operating Activities:
Depreciation	116,519	105,372	98,436
Deferred Income Taxes	87,345	83,776	94,813
Pension, SERP and PBOP Expense	875	4,580	7,197
Pension and PBOP Contributions	(17,078)	(982)	(2,482)
Regulatory Over/(Under) Recoveries, Net	(4,491)	41	(11,875)
Amortization of Regulatory Assets/(Liabilities), Net	11,170	16,276	(29,602)
Refunds/(Payments) Related to Spent Nuclear Fuel, Net	3,926	979	14,453
Other	6,521	8,677	10,095
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	(18,822)	(4,750)	(15,576)
Fuel, Materials, Supplies and Inventory	(5,485)	(8,729)	(19,403)
Taxes Receivable/Accrued, Net	32,303	(23,909)	(23,857)
Accounts Payable	11,353	(22,203)	17,796
Other Current Assets and Liabilities, Net	5,651	953	(5,972)
Net Cash Flows Provided by Operating Activities	361,772	274,523	247,967
Investing Activities:
Investments in Property, Plant and Equipment	(305,430)	(308,036)	(256,159)
Other Investing Activities	326	306	(1,152)
Net Cash Flows Used in Investing Activities	(305,104)	(307,730)	(257,311)
Financing Activities:
Cash Dividends on Common Shares	(77,600)	(106,000)	(66,000)
(Decrease)/Increase in Notes Payable to Eversource Parent	(70,400)	140,800	4,000
Issuance of Long-Term Debt	0	0	75,000
Retirements of Long-Term Debt	0	0	(50,000)
Capital Contributions from Eversource Parent	94,500	0	45,000
Other Financing Activities	(255)	(349)	1,703
Net Cash Flows (Used in)/Provided by Financing Activities	(53,755)	34,451	9,703
Net Increase/(Decrease) in Cash and Cash Equivalents	2,913	1,244	359
Cash and Cash Equivalents - Beginning of Year	1,733	489	130
Cash and Cash Equivalents - End of Year	4,646	1,733	489
Western Massachusetts Electric Company [Member]
Operating Activities:
Net Income	58,072	56,506	57,819
Provided by Operating Activities:
Depreciation	46,147	43,362	41,886
Deferred Income Taxes	21,498	39,428	34,108
Regulatory Over/(Under) Recoveries, Net	590	(17,501)	1,925
Amortization of Regulatory Assets/(Liabilities), Net	4,249	14,545	(6,228)
Refunds/(Payments) Related to Spent Nuclear Fuel, Net	3,553	(56,784)	18,883
Other	(3,726)	(6,421)	(2,005)
Changes in Current Assets and Liabilities:
Receivables and Unbilled Revenues, Net	(7,164)	(17,822)	39,872
Taxes Receivable/Accrued, Net	33,340	(15,281)	(22,454)
Accounts Payable	(1,915)	(2,602)	1,269
Other Current Assets and Liabilities, Net	12,686	5,594	(11,796)
Net Cash Flows Provided by Operating Activities	167,330	43,024	153,279
Investing Activities:
Investments in Property, Plant and Equipment	(140,626)	(134,551)	(116,205)
Proceeds from Sales of Marketable Securities	2,479	186,444	73,198
Purchases of Marketable Securities	(2,426)	(128,861)	(73,888)
Other Investing Activities	0	0	3,200
Net Cash Flows Used in Investing Activities	(140,573)	(76,968)	(113,695)
Financing Activities:
Cash Dividends on Common Shares	(38,000)	(37,200)	(59,999)
(Decrease)/Increase in Notes Payable to Eversource Parent	(92,400)	122,000	21,400
Issuance of Long-Term Debt	50,000	0	0
Retirements of Long-Term Debt	0	(50,000)	0
Capital Contributions from Eversource Parent	53,000
Other Financing Activities	(191)	(22)	(985)
Net Cash Flows (Used in)/Provided by Financing Activities	(27,591)	34,778	(39,584)
Net Increase/(Decrease) in Cash and Cash Equivalents	(834)	834	0
Cash and Cash Equivalents - Beginning of Year	834	0	0
Cash and Cash Equivalents - End of Year	$ 0	$ 834	$ 0