Revenues (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011
Schedule 21-NU Wholesale Transmission Revenue Rates Overrecovery/(Underrecovery) Position	$ 31.4
The Connecticut Light And Power Company [Member]
Schedule 21-NU Wholesale Transmission Revenue Rates Overrecovery/(Underrecovery) Position	18.6
Public Service Company Of New Hampshire [Member]
Schedule 21-NU Wholesale Transmission Revenue Rates Overrecovery/(Underrecovery) Position	1.7
Western Massachusetts Electric Company [Member]
Regulated Operating Revenue, Decoupled Distribution Revenues	125.6
Schedule 21-NU Wholesale Transmission Revenue Rates Overrecovery/(Underrecovery) Position	$ 11.1