LONG-TERM DEBT (Details) (USD $) In Millions	9 Months Ended
Sep. 30, 2011
Public Service Company of New Hampshire [Member] \| Series Q 2011 Due 2021 4.05% [Member]
Debt Instrument Line Items
Long-term Debt	$ 122.0
Long-term Debt, Description	On May 26, 2011, PSNH issued $122 million of Series Q first mortgage bonds with a coupon rate of 4.05 percent and a maturity date of June 1, 2021. The proceeds of these bonds were used to redeem two series of tax-exempt PCRBs. The indenture under which the bonds were issued requires PSNH to comply with certain covenants as are customarily included in such indentures.
Public Service Company of New Hampshire [Member] \| Series R 2011 Due 2021 3.20% [Member]
Debt Instrument Line Items
Long-term Debt	160.0
Long-term Debt, Description	On September 13, 2011, PSNH issued $160 million of Series R first mortgage bonds with a coupon rate of 3.20 percent and a maturity date of September 1, 2021. The proceeds, net of issuance expenses, were used to repay short-term borrowings previously incurred in the ordinary course of business and for general capital purposes. The indenture under which the bonds were issued requires PSNH to comply with certain covenants as are customarily included in such indentures.
Western Massachusetts Electric Company [Member] \| Series F 2011 Due 2021 3.50% [Member]
Debt Instrument Line Items
Long-term Debt	$ 100.0
Long-term Debt, Description	On September 16, 2011, WMECO issued $100 million of Series F unsecured senior notes with a coupon rate of 3.50 percent and a maturity date of September 15, 2021. The proceeds, net of issuance expenses, were used to repay short-term borrowings previously incurred in the ordinary course of business. The indenture under which the notes were issued requires WMECO to comply with certain covenants as are customarily included in such indentures.

On May 26, 2011, PSNH issued $122 million of Series Q first mortgage bonds with a coupon rate of 4.05 percent and a maturity date of June 1, 2021. The proceeds of these bonds were used to redeem two series of tax-exempt PCRBs. The indenture under which the bonds were issued requires PSNH to comply with certain covenants as are customarily included in such indentures.

Long-term Debt, Description

On September 13, 2011, PSNH issued $160 million of Series R first mortgage bonds with a coupon rate of 3.20 percent and a maturity date of September 1, 2021. The proceeds, net of issuance expenses, were used to repay short-term borrowings previously incurred in the ordinary course of business and for general capital purposes. The indenture under which the bonds were issued requires PSNH to comply with certain covenants as are customarily included in such indentures.

Debt Instrument Line Items

Long-term Debt, Description

On September 16, 2011, WMECO issued $100 million of Series F unsecured senior notes with a coupon rate of 3.50 percent and a maturity date of September 15, 2021. The proceeds, net of issuance expenses, were used to repay short-term borrowings previously incurred in the ordinary course of business. The indenture under which the notes were issued requires WMECO to comply with certain covenants as are customarily included in such indentures.