PSNH GENERATION ASSET SALE - Narrative (Details) $ in Millions	Oct. 11, 2017 USD ($) agreement	Jun. 10, 2015 USD ($) senator
Regulatory Assets [Line Items]
Equity return related to clean air project forgone		$ 25
Electricity Generation Plant, Non-Nuclear | Public Service Company Of New Hampshire
Regulatory Assets [Line Items]
Number of state senators | senator		2
Subsequent Event
Regulatory Assets [Line Items]
Number of purchase sale agreements | agreement	2
Thermal generation assets	$ 175
Hydroelectric assets	$ 83