SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - BALANCE SHEET (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current Assets:
Cash	$ 30,251	$ 23,947	$ 38,703	$ 43,364
Prepayments and Other Current Assets	134,813	129,034
Total Current Assets	2,477,672	2,618,786
Deferred Debits and Other Assets:
Investments in Subsidiary Companies, at Equity	236,900	48,000
Goodwill	3,519,401	3,519,401
Other Long-Term Assets	522,260	295,243
Total Deferred Debits and Other Assets	8,224,991	8,069,082
Total Assets	32,053,173	30,580,309
Current Liabilities:
Notes Payable	1,148,500	1,160,953
Long-Term Debt – Current Portion	773,883	228,883
Accounts Payable	884,521	813,646
Other Current Liabilities	684,914	678,549
Total Current Liabilities	3,638,605	2,989,790
Total Deferred Credits and Other Liabilities	8,717,912	8,277,162
Capitalization:
Long-Term Debt	8,829,354	8,805,574
Common Shareholders' Equity:
Common Shares	1,669,392	1,669,313
Capital Surplus, Paid In	6,250,224	6,262,368
Retained Earnings	3,175,171	2,797,355
Accumulated Other Comprehensive Loss	(65,282)	(66,844)
Treasury Stock	(317,771)	(309,977)
Common Shareholders' Equity	10,711,734	10,352,215	9,976,815	9,611,528
Total Capitalization	19,696,656	19,313,357
Total Liabilities and Capitalization	32,053,173	30,580,309
Eversource [Member]
Current Assets:
Cash	93	67	$ 138	$ 35
Accounts Receivable from Subsidiaries	32,864	23,689
Notes Receivable from Subsidiaries	740,300	850,300
Prepayments and Other Current Assets	23,122	41,254
Total Current Assets	796,379	915,310
Deferred Debits and Other Assets:
Investments in Subsidiary Companies, at Equity	9,703,287	8,915,178
Notes Receivable from Subsidiaries	224,290	128,800
Accumulated Deferred Income Taxes	126,091	143,054
Goodwill	3,231,811	3,231,811
Other Long-Term Assets	44,020	48,314
Total Deferred Debits and Other Assets	13,329,499	12,467,157
Total Assets	14,125,878	13,382,467
Current Liabilities:
Notes Payable	1,022,000	1,098,453
Long-Term Debt – Current Portion	28,883	28,883
Accounts Payable	0	78
Accounts Payable to Subsidiaries	8,771	15,601
Other Current Liabilities	47,215	60,999
Total Current Liabilities	1,106,869	1,204,014
Total Deferred Credits and Other Liabilities	148,756	134,908
Capitalization:
Long-Term Debt	2,158,519	1,691,330
Common Shareholders' Equity:
Common Shares	1,669,392	1,669,313
Capital Surplus, Paid In	6,250,224	6,262,368
Retained Earnings	3,175,171	2,797,355
Accumulated Other Comprehensive Loss	(65,282)	(66,844)
Treasury Stock	(317,771)	(309,977)
Common Shareholders' Equity	10,711,734	10,352,215
Total Capitalization	12,870,253	12,043,545
Total Liabilities and Capitalization	$ 14,125,878	$ 13,382,467