SCHEDULE I - FINANCIAL INFORMATION OF REGISTRANT - BALANCE SHEET (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Current Assets:
Cash	$ 38,165	$ 30,251	$ 23,947	$ 38,703
Prepayments and Other Current Assets	138,009	215,284
Total Current Assets	2,487,099	2,477,672
Deferred Debits and Other Assets:
Investments in Subsidiary Companies, at Equity	277,600	236,900
Goodwill	4,427,266	3,519,401
Other Long-Term Assets	605,692	522,260
Total Deferred Debits and Other Assets	10,115,824	8,224,991
Total Assets	36,220,386	32,053,173
Current Liabilities:
Notes Payable	1,088,087	1,148,500
Long-Term Debt – Current Portion	549,631	773,883
Accounts Payable	1,085,034	884,521
Other Current Liabilities	738,222	684,914
Total Current Liabilities	3,589,045	3,638,605
Total Deferred Credits and Other Liabilities	9,613,640	8,717,912
Capitalization:
Long-Term Debt	11,775,889	8,829,354
Common Shareholders' Equity:
Common Shares	1,669,392	1,669,392
Capital Surplus, Paid in	6,239,940	6,250,224
Retained Earnings	3,561,084	3,175,171
Accumulated Other Comprehensive Loss	(66,403)	(65,282)
Treasury Stock	(317,771)	(317,771)
Common Shareholders' Equity	11,086,242	10,711,734	10,352,215	9,976,815
Total Capitalization	23,017,701	19,696,656
Total Liabilities and Capitalization	36,220,386	32,053,173
Eversource
Current Assets:
Cash	521	93	$ 67	$ 138
Accounts Receivable from Subsidiaries	3,397	32,864
Dividends Receivable from Subsidiary	150,000	0
Notes Receivable from Subsidiaries	844,500	740,300
Prepayments and Other Current Assets	18,568	23,122
Total Current Assets	1,016,986	796,379
Deferred Debits and Other Assets:
Investments in Subsidiary Companies, at Equity	10,945,986	9,703,287
Notes Receivable from Subsidiaries	312,190	224,290
Accumulated Deferred Income Taxes	47,940	126,091
Goodwill	3,231,811	3,231,811
Other Long-Term Assets	58,313	44,020
Total Deferred Debits and Other Assets	14,596,240	13,329,499
Total Assets	15,613,226	14,125,878
Current Liabilities:
Notes Payable	778,087	1,022,000
Long-Term Debt – Current Portion	32,114	28,883
Accounts Payable	292	0
Accounts Payable to Subsidiaries	18,242	8,771
Other Current Liabilities	56,601	47,215
Total Current Liabilities	885,336	1,106,869
Total Deferred Credits and Other Liabilities	118,176	148,756
Capitalization:
Long-Term Debt	3,523,472	2,158,519
Common Shareholders' Equity:
Common Shares	1,669,392	1,669,392
Capital Surplus, Paid in	6,239,940	6,250,224
Retained Earnings	3,561,084	3,175,171
Accumulated Other Comprehensive Loss	(66,403)	(65,282)
Treasury Stock	(317,771)	(317,771)
Common Shareholders' Equity	11,086,242	10,711,734
Total Capitalization	14,609,714	12,870,253
Total Liabilities and Capitalization	$ 15,613,226	$ 14,125,878