Fuel, Materials and Supplies and Allowance Inventory (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Public Utilities Inventory Fuel [Member]
Other Inventory, Current	$ 152,500	$ 164,300
Public Utilities Inventory Supplies [Member]
Other Inventory, Current	131,200	159,500
Emission Allowances [Member]
Other Inventory, Current	1,900	100
Other Inventory, Noncurrent	17,500	20,100
RECs [Member]
Other Inventory, Current	50,900	25,800
NSTAR Electric Company [Member]
Materials and Supplies	75,487	74,179
NSTAR Electric Company [Member] | Public Utilities Inventory Supplies [Member]
Other Inventory, Current	32,200	49,100
NSTAR Electric Company [Member] | RECs [Member]
Other Inventory, Current	43,300	25,100
Public Service Company Of New Hampshire [Member] | Public Utilities Inventory Fuel [Member]
Other Inventory, Current	103,400	95,100
Public Service Company Of New Hampshire [Member] | Public Utilities Inventory Supplies [Member]
Other Inventory, Current	44,600	52,200
Public Service Company Of New Hampshire [Member] | Emission Allowances [Member]
Other Inventory, Current	1,900	100
Other Inventory, Noncurrent	17,500	20,100
Public Service Company Of New Hampshire [Member] | RECs [Member]
Other Inventory, Current	$ 7,000	$ 700