Other Contingencies (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2013
FERC ROE Reserve - First Complaint - Base ROE [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	$ 24.9	$ 1.2	$ 23.7
FERC ROE Reserve - First Complaint - Base ROE [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	13.3	0.5	12.8
FERC ROE Reserve - First Complaint - Base ROE [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	6.1	0.4	5.7
FERC ROE Reserve - First Complaint - Base ROE [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	2.4	0.1	2.3
FERC ROE Reserve - First Complaint - Base ROE [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	3.1	0.2	2.9
FERC ROE Reserve - Second Complaint - Base ROE [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	27.4	27.4
FERC ROE Reserve - Second Complaint - Base ROE [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	13.5	13.5
FERC ROE Reserve - Second Complaint - Base ROE [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	7.5	7.5
FERC ROE Reserve - Second Complaint - Base ROE [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	2.7	2.7
FERC ROE Reserve - Second Complaint - Base ROE [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	3.7	3.7
FERC ROE Reserve - Incentive - First and Second Complaint [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	23.8	23.8
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	16.1	16.1
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	2.0	2.0
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	0.8	0.8
FERC ROE Reserve - Incentive - First and Second Complaint [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	4.9	4.9
FERC ROE Reserve - Cumulative [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	76.1	52.4	23.7
FERC ROE Reserve - Cumulative [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	42.9	30.1	12.8
FERC ROE Reserve - Cumulative [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	15.6	9.9	5.7
FERC ROE Reserve - Cumulative [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	5.9	3.6	2.3
FERC ROE Reserve - Cumulative [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss	11.7	8.8	2.9
FERC ROE Reserve - Cumulative - After Tax Charge [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss		32.1	14.3
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | The Connecticut Light And Power Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss		18.5	7.7
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | NSTAR Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss		6.1	3.4
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | Public Service Company Of New Hampshire [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss		2.0	1.4
FERC ROE Reserve - Cumulative - After Tax Charge [Member] | Western Massachusetts Electric Company [Member]
Loss Contingencies Line Items
Loss Contingency Estimate Of Possible Loss		$ 5.5	$ 1.8