PSNH GENERATION ASSET SALE - Narrative (Details) $ in Thousands		12 Months Ended
	Jun. 10, 2015 USD ($) senator	Dec. 31, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2014 USD ($)
Regulatory Assets [Line Items]
Equity return related to clean air project forgone	$ 25,000
Other current liabilities associated with PSNH's generation assets		$ 678,549	$ 684,914
Asset retirement obligation associated with PSNH's generation assets		430,100	426,400	$ 426,300
Accrued pension, SERP and PBOP associated with PSNH's generation assets		1,407,288	1,141,514
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Other current liabilities associated with PSNH's generation assets		43,921	43,253
Asset retirement obligation associated with PSNH's generation assets		21,600	23,500	$ 20,600
Accrued pension, SERP and PBOP associated with PSNH's generation assets		89,579	94,652
Electricity Generation Plant, Non-Nuclear [Member] | Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Number of state senators | senator	2
Payments for restructuring	$ 5,000	$ 5,000
Accounts payable associated with PSNH's generation assets			40,500
Other current liabilities associated with PSNH's generation assets			16,100
Asset retirement obligation associated with PSNH's generation assets			20,000
Accrued pension, SERP and PBOP associated with PSNH's generation assets			$ 24,300