Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Capital Expenditures Incurred But Not Yet Paid	$ 216.6	$ 181.9	$ 193.1
Interest Paid, Net of Amounts Capitalized	365.9	349.6	343.3
Income Taxes Paid	10.3	334.2	50.0
The Connecticut Light And Power Company [Member]
Capital Expenditures Incurred But Not Yet Paid	76.0	63.5	51.4
Interest Paid, Net of Amounts Capitalized	144.4	144.1	131.6
Income Taxes Paid	55.2	135.4	55.0
NSTAR Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	23.5	34.6	57.0
Interest Paid, Net of Amounts Capitalized	75.7	75.3	75.8
Income Taxes Paid	(19.8)	217.1	163.4
Public Service Company Of New Hampshire [Member]
Capital Expenditures Incurred But Not Yet Paid	46.5	39.3	34.9
Interest Paid, Net of Amounts Capitalized	42.3	41.1	43.3
Income Taxes Paid	14.4	2.3	(30.1)
Western Massachusetts Electric Company [Member]
Capital Expenditures Incurred But Not Yet Paid	27.0	14.2	19.5
Interest Paid, Net of Amounts Capitalized	26.7	25.9	25.8
Income Taxes Paid	$ 14.7	$ 25.1	$ (69.0)