PSNH GENERATION ASSET SALE - Narrative (Details) $ in Thousands	Jun. 10, 2015 USD ($) senator	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Regulatory Assets [Line Items]
Equity return related to clean air project forgone	$ 25,000
Other current liabilities		$ 639,366	$ 684,914
Accrued Pension and SERP		1,077,593	1,141,514
Public Service Company Of New Hampshire [Member]
Regulatory Assets [Line Items]
Other current liabilities		52,771	43,253
Accrued Pension and SERP		91,911	$ 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
Accounts payable		30,100
Other current liabilities		21,700
Asset retirement obligation		20,300
Accrued Pension and SERP		$ 23,700